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MG MGB Technical - Pressure diff switch

If you have been following the thread of my previous adventures with the brake system you know that I broke the plastic pressure differential switch (PDS)on the bottom of the brake master cylinder. I was attempting to bleed the brakes, and with the rubber bumper cars you first need to loosen the PDS three and one half turns. So the simple task of bleeding the brakes has taken a new twist...I need to replace the PDS.
I looked up the part in Moss. $34 to get another. It's a small piece of plastic, with 3/8" fine threads. I tried plugging the hole with a steel bolt of the same size and I couldn't get any brake pressure when I tried to bleed the brakes.
So I called Moss and the sales rep said a lot of people just plug the hole rather than replace the rather pricey switch, which was disconnected by the previous owner anyway and the original wires are no where in sight.
The question is: Has anyone successfully plugged the hole where the PDS is supposed to go, and if so, how was the hole plugged?
Regards,
Randy
1977 MGB
PS Thanks to Clifton, Les, and all who helped me in the previous thread.
randy olson

Randy. Just wrote a long discussion of this, then, hit the tab button by accident and the whole thing went into cyber-space.

Short version--yes, I have plugged the hole using either a 1/4" UNF or a 5/16" UNF bolt of about 3/8" length. I used some non-hardening Form-A-Gasket on the bolt threads. I was able to successfully bleed the brake system and it worked well for several years. Eventually, the old master cylinder was replaced with a new one. The old bolt was installed into the new MC and is working well today.

The rubber O rings on the balance rod should keep air out of the system. If it does not, there is a problem with the working of the master cylinder. Replacing the switch with a bolt is a "work around" solution. But, the best solution is to rebuild the MC or replace it with a new one.

Were I going to insert a bolt today, I would remove the MC, clean the area throughly with brake cleaner, use some J-B Weld on the proper sized bolt and let it harden for a day before re-installing the MC and bleeding it.

Am bringing a 79 on line right now. Just picked up the new MC yesterday. No need to worry about having the pitted bore of the old MC sleeved (with erratic success), nor how many times it may have been rebuilt over the years. I am staring off with a known good quantity and, if it is not, it will be replaced under warranty.

Les
Les Bengtson

Randy, can't help with using a bolt I am afraid. I ended up getting a replacement switch. To my surprise this is actually a switch. The old one was just a pin in a hunk of plastic! I need to pull the master cylinder apart though (well, at least the pressure failure bit) to see why it is leaking now. My job for the weekend I think. I think in NZ it is actually illegal to not use the switch if one was originally fitted although I am sure if I put a bolt in no one would be any wiser!

Simon
Simon Jansen

Randy,
The switch serves 2 purposes. 1-to warn of loss of fluid in one half of the braking system. 2- to lightly hold the brake warning shuttle in the middle of its travel. If you replace the switch with a bolt the shuttle will move about in an uncontrolled manner and could lead to failure of both halves of the switch at once which could be rather embarassing!
You unscrew the switch when bleeding to prevent the shuttle breaking the switch as it moves sideways due to unequal pressure in the system.
Get a switch the same as the original, even a secondhand one from a swap meet. The little metal spring-loaded pin is the crucial part.
Greg
Greg Taplin

Well, I just found a 1/2" long bolt that should work. It's 3/8" fine thread stainless steel. This fits the threads of the hole in the master cylinder.
The original plastic sensor has a groove from the end of the threads to the underside of the hex cap. Is this significant? In other words, do I need to groove the new bolt, if I use it? The sensor also tapers, that is, it has a smaller diameter at the end...is this important?
If the sensor, or bolt, is removed from the master cylinder completely are you able to bleed the brakes? I still haven't been able to achieve any pressure in the brake lines.
Since I don't have a warning light connected to the sensor that part is not important to me. The other issue of stabilizing the rod in the master cylinder seems legitimate, and I will need to consider it.
Randy
randy olson

Randy, just guessing here but I think that the slot is there so that the shuttle chamber can breath instead of building pressure do to temp changes. A tiny hole drilled thru the bolt should perform the same function.
John H

Randy. Sorry to take so long in following up. Yes, you are perfectly correct, the thread is the 3/8" UNF (3/8"-24 from memory).

Let us look at some of Greg's comments. The flow pattern of the hydraulic fluid is from the reservoir down into the main shaft of the master cylinder. As the brake pedal is moved forwards, the brake fluid is pressed out of the main bore and through two small openings in the secondary bore where the "brake failure" switch resides. Thus, if the secondary bore is not perfect, and the O ring seals become torn, the fluid will exit via the warning switch and air will enter the system as the brake pedal is released. This is the problem I had with my daughter's car. The fluid was leaking from the switch (three "master cylinder rebuilds" by the MG Shoppe in Tempe, Arizona had not corrected this problem prior to our purchase of the car). Thus, replacing the switch with a 3/8" UNF bolt, with a little Form A Gasket number 2, corrected the problem.

There are a couple of companies which will re-sleeve the main bore of the 77-80 master cylinder. None of them write of what they will do to the "secondary bore of the master cylinder" where the "brake warning light switch" rides.

There are two areas which might allow air to enter. First is the switch, which is open to the atomosphere. The second is the nut on the front of the master cylinder which, if it does not seal properly, allows air to be drawn in. The switch has no real seal. The front bolt has a copper seal which needs to be annealled, or replaced, during a rebuild.

Plugging the warning light switch hole with a bolt will overcome the problem of a leaking O ring in the warning light balance system.

Is this wise? A personal decision. In the case of my daughter's car, the problem was in the O rings of that part of the system or in the bore which was causing the O rings not to function properly. Installing the bolt allowed a proper seal to be made and the system worked as it should.

What is lost? My experience is that nothing is lost. The two times I have experienced a real brake system failure, I knew it before the light came on (or before I noticed the light had come on). Three other times I have had the light come on, have not noted a failure in how the brakes felt, then have taken the various cars to professional mechanics for evaluation. In none of those cases could the professionals find a problem with the brake system.

Thus, my experience is, if you have a real problem, you will notice it, immediately, when braking. The warning light my come on where there is a minor imbalance in the system, but when the system is working well and, when evaluated by a professional mechanic, no problem will be found.

Thus, the decision to plug off the failure warning light system is a personal one, but, one which I am comfortable with. If you are not confortable with this, Moss has new master cylinders available and several of the Moss distributors, such as Jeff Zorn's Little British Car Company, can get them for you at a discount.

I have replaced most of the master cylinders on my cars with new ones. To me, this makes good sense. Rather than dealing with a master cylinder that is, at least 26 years old, and with an unknown history of rebuilds, I can install a new master cylinder and keep track of it during my lifetime. But, I am stil inclined to install a bolt into the hole for the brake failure warning switch because I know that the switch itself can be a cause of brake failure.

Les
Les Bengtson

I have two bolts of the same size to -plug the switch hole. One has a 3/32" hole drilled through the center to equalize air pressure. The other is solid. I can use either one at this point, and have no qualms about losing a pressure switch that was not even connected for the last ten years. Several people, including a sales rep at a major MG parts distributor, said a lot of people ditch the switch.
One of the things someone suggested is that there was an air lock that prevented fluid from getting to the calipers. This may be associated with a need to prime the system before bleeding. I'm not sure how this is done, but it seems likely you would need to introduce fluid into the master cylinder bore so that it would draw fluid from the reservoir as the pedal was slowly depressed and the normal bleeding procedures followed. We have all heard of priming the pump in reference to water, but it might also apply to the brake master cylinder.
Any thoughts?
Randy
1977 MGB
randy olson

Randy, Les was kind enough to point out tm me in the other thread that he has a link on his site to an article about bench bleeding the system: www.custompistols.com/ under the MG section, then click on articles.

I'd already done mine by the time I saw the post from Les. I had trouble on mine with getting the rear brakes to bleed. What I did was hold a piece of vinyl tube tight in the outlet on the master and then carefully sucked on the other end of the tube until fluid came out. I used a nice long bit of tube since ingesting brake fluid seemed to be a bad idea! Once I did that fluid would pump from the master when I pumped the pedal.

I'd sugest having a look at the article Les has.

Simon
Simon Jansen

Simon,
I can try your method. In fact I could probably even use this vacuum pump called a MityVac to get the same results without risk of swallowing a dose of brake fluid.
I am getting good flow to the front calipers, but absolutely no fluid at the rears. That's after at least 15 pumps. Something is not right. I even tried disconnecting the brake pipe to the rear cylinders and there was still no fluid emerging from the pipe.
Thanks for the input. I'll check it now.
Randy
1977 MGB
randy olson

I just checked to see if I could suck any fluid out of the master cylinder outlet to the rear brakes and couldn't get anything but air. I tried with both the bolt with a 3/32" hole through it, and with a unvented bolt. I used the MityVac and it couldn't get any pressure. There must be a leak in the M/C preventing the buildup of pressure...I'm just guessing here.
Maybe the reservoir outlet to the rear brake is plugged.
Maybe the pressure sensor does something the bolt doesn't.
Correct me if I'm wrong, but the rear brakes are connected to the outlet on the front of the M/C, right?
Let me know if you can think of anything that I might try.
Randy
randy olson

Randy-
I think maybe you have the same trouble as Gdawg - "No rear brakes" on the Spridget board. There is a connector link between the two MC pistons, which wears out/breaks. Without this there is no way for the rear (actually front or bottom - damn, the first one in the bottom of the bore!) piston to retract and suck fluid in, so it can't push any out. Part is NOT available; I have made a couple of these but it is difficult, and the MC is usually not worth the effort anyway. Send it back as a core, or buy new.
FRM
FR Millmore

I just left a message on the Spridget board to see if anyone there had any ideas about the problem with the rear brakes not bleeding:
It started when I needed to replace a leaking rear wheel brake cylinder. After replacing both rear cylinders and shoes, I tried to bleed the brakes.
I didn't back out the pressure sensor on the master cylinder, so the brakes wouldn't bleed. Then when I unscrewed the sensor, the plastic sensor broke. So I took the master cylinder off and removed the sensor. Now there's a bolt of the same size in it's place.
The problem is that there is no brake fluid going to the rear brakes. I took off the brake pipe at the master cylinder and tried sucking out fluid to prime things and get the flow of fluid started, but I can only get air.
The reservoir is full on the front brake side, but I noticed that the rectangular reservoir is empty over the intake port at the front...is this right?
The front calipers bleed normally. The rears not at all.
Before I buy a new master cylinder I want to make sure I'm not missing something obvious, or that there isn't a simpler fix.
Let me know if you have any ideas.
Regards,
randy olson

To the best of my knowledge the front reservoir is the one that supplies fluid to the rear brakes...it should fill as you fill the MC after the rear chamber fills and begins to overflow into it. Is this happening? If not is your front end jacked up such that it is significantly higher than the rear end?
Frank

Well, let me say THANKS to all that contributed to another successful adventure into car mechanics! I am here to report that the brakes now work...and work well.
A brief history: A leaking rear brake cylinder caused me to replace both rear cylinders and the shoes. Then I broke the pressure sensor on the underside of the master cylinder after learning that it needed to be loosened to bleed the brakes. That sensor was replaced with a 3/8" fine thread, stainless steel bolt, 1/2" long. In the process of all this I removed the master cylinder (in retrospect, I can't logically say why!). So, the fluid in the reservoir was drained. After replacing the master cylinder and trying to bleed the brakes, only the front brakes calipers would bleed normally...the backs only had air coming out. After realizing the reservoir is actually two separate compartments, one for the front and one for the back, and filling the one for the back brakes (which is on the front-of-the-car side of the reservoir), I was able to successfully bleed the brakes with my wife pumping the pedal and going through the normal routine...voila! Topped off the reservoir, tightened the new stainless steel sensor-replacement, and took it for a test drive. It now stops better than ever.
Again, thanks to everyone involved with the thread. Stayed tuned for my future intrepid travels in the world of British car maintenance!
Randy
1977 MGB with twin SUs
1969 E-Type Jaguar (currently without brakes)
randy olson

I am surprised that some one has not mentioned the main reason for a duel circuit brake system. Namely to have one system work (front or rear in the case of a rear wheel drive car) when the other system fails.
When one system starts to get weak the shuttle valve moves along and shuts off that system and allows full pressure to the remaining good system. The light comes on. The little pin in the switch you just threw away holds it there until the brakes are repaired and the valve reset by removing and replacing the switch. The brakes will work great without the switch, but you have lost the reliability of a duel circuit system, and in our country at least unroadworthy. Even if the switch is unwired, providing it is fitted the brakes will work as a duel system. Not trying to frighten anyone, just making you aware. Because removing the switch makes the brakes act as a single MC you can see why it makes it possible to bleed the bakes, other wise when you open a bleeder the shuttle valve thinks the system has failed and shuts it off.
Happy stopping. Denis
DENIS H

Hmmm, Denis, that little switch is still on the bench, but I could buy another and just stick it in there.
Didn't the older MGBs have a plain master cylinder without the dual system and work fine?
Randy
randy olson

Denis is right about the later switches also holding the shuttle in place once one side fails. That's why you unscrew it when bleeding, so it doesn't lock the shuttle over to one side.

But not all of them were like that. I posted pics of mine in the other brake switch thread. You can see the difference in the shuttles and the pins between the locking and no locking versions.

Simon
Simon Jansen

Because of the comments of Denis and Greg, I today milled away part of a 79 brake master cylinder--the portion which holds the spring and shuttle assembly which activates the brake failure warning light system. This is the later design which, as Simon notes, is designed to have a switch which is normally in the "on" position. It is held off by the center rib of the shuttle. When there is a pressure differential, the shuttle moves, allowing the shuttle to move away from the higher pressure source, the spring in the switch to force the plunger upwards and lock the shuttle with the switch now transmitting electricity to the brake failure warning light.

I found some things which might be of interest. First, the hole, coming in from the front and plugged by the bolt and copper washer is of two diameters. The front, major diameter, is sufficiently large to accept the spring around the front of the shuttle. The spring is secured at both ends with a "C" clip. The rearward portion of the hole, the minor diameter, is only slightly larger than the rear end of the shuttle and provides just sufficient space for the O rings to sit. (Both O rings are contained within the minor diameter.)

Both sections of the hole showed rust and pitting. (Such is probably what caused the damage to the O rings, allowing fluid to leak out and air to be drawn into the system.) It would be very hard to clean up any rust or minor pitting due to the small size and depth of the bore. Thus, on any master cylinder showing pitting in the main bore, one should assume that there may be pitting secondary (pressure differential switch) bore and that there is a chance of O ring damage during the bleeding process.

The movement of the shuttle is controlled under spring tension when at rest. When there is excess pressure at the rear (front brakes) forward movement is stopped by coil bind of the spring. When there is excess pressure at the front (rear brakes) rearwards movement is controlled by the shuttle hitting the rear (bottom) of the hole. In neither case can the shuttle move sufficiently to allow the O rings to be moved into the switch area, even if the switch is not present to prevent such movement. Thus, not having the switch in place will have little effect on the system should the system experience a pressure differential in one section of the system.

Les
Les Bengtson

This thread was discussed between 27/04/2006 and 04/05/2006

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