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MG MGB Technical - Where is my brake fluid going?

I slowly, over the course of a few hours of driving, lose brake fluid in the back reservoir of my master cylinder on my 74.5, dual circuit, no booster, "B". Every other push of my brake pedal goes to the floor also. I have looked the system over closely and see no leaks except the slight hint of moisture on the master cylinder shaft that hooks to the foot pedal and this looks old. In other words, I have examined the rear wheel cylinders, front calipers, rubber hose junctions (these were replaced a few months ago), the five I/O valve and the master. There is no fluid on the floorboard at the driver's feet either. I don't know where the fluid is going but I need to get the brakes working correctly so I am going to start with a new master cylinder. Any ideas or advice?
Tom GIllett

Tom,

Have you looked at the steel lines that run down the firewall and then under the car to see if they have rusted through? Look also where they pass through the rubber anti-chafe blocks.

Maybe you can have someone operate the brakes while you watch under the car. That much brake fluid should show up somewhere since you don't have a booster to feed it to the engine.

Charley
C R Huff

did you check the rear axle hose/connection?
people tend to forget abut that one. I do.
Peter Sherman

Mine ended up in the booster ,which had to be replaced. Take the vacuum hose off & use a bit of wire for a dipstick. Barrie E
Barrie Egerton

How's the fluid level in the reservoir?

If it's unchanged, and the brake pedal still drifts to
the floor when pressed - then the leak is wholly
internal, and there are no outward signs.

In other words, fluid is squishing back & forth
past the internal piston seals - but not externally.
Daniel wong

"I ... lose brake fluid in the back reservoir"

"no booster"

!!!!
Paul Hunt 2

With no booster and only the back chamber drawing down, it seems the front system needs to be checked. Doesn't the back chamber feed the front wheels?

Daniel, could the internals of the MC draw fluid from the rear, and not slosh it back? If so, Tom should see a corresponding increase in the front chamber fluid level.

I guess I would start with the master cylinder, and carefully bleed the system, and see what happens.

hope this helps,
dave
Dave Braun

The back reservoir *does* feed the front calipers - *if* things have been connected the right way round.

And yes it is possible for fluid to travel between the circuits, via the shuttle valve for the brake pressure failure switch, whether in the earlier remote manifold or as part of the later master with servo. However it would require two faults - a leak in the rear circuit *and* a leak in the shuttle valve that always transferred the correct amount of fluid from the front circuit (rear reservoir) to the other. Possible, but only just, I suspect.
Paul Hunt 2

Are you sure the back reservoir feeds the front brakes? I'm asking because on my dual master cylinder 74 the rear cylinder pipe goes to the rear section of the brake pressure failure valve and to the rear brakes. I have not changed the plumbing and it appears to be unmolested OE steel tubing. I have removed the brake pressure failure valve and the master cylinder on separate occasions and the pipes will only fit with the rear pipe to the rear and front pipe to the front, they easily fit without any pulling or bending. I didn't buy the car new, but it's difficult to bend those pipes with damaging them. I used to own a 68 GT and it also had the rear brakes connected to the rear reservoir.

In this Moss drawing the rear master cylinder pipe (#18) is shown going to the rear brakes through the rear of the brake pressure failure switch. The front pipe (#17) is shown feeding the front brakes.
http://www.mossmotors.com/Shop/ViewProducts.aspx?PlateIndexID=29035
I realize the drawing could be wrong, but could I be so unlucky that I have owned two cars that had the brake plumbing switched? What would be the point in switching?

This discussion matters if Tom's brakes are plumbed as they are on my car he needs to check the rear brakes.

Clifton
Clifton Gordon

Clifton,

Admitedly, I use the Haynes manual, but it shows the back reservoir feeding the front brakes. The only thig I use the Moss catalog for is 1) ordering parts and 2) reading the stuff on the electrical wiring scheme and the associated colors. The rest is often drawn for convenience, not accuracy.

It would be a simple mistake for someone to install new brake lines and plumb the MC backwards.

warm regards,
dave
Dave Braun

The drawings in the Haynes manual are for the later cars with a servo. I believe Tom has the MC as used from 68-74. They use different master cylinders and I'm not familiar with how the later cars are plumbed.

I just have a hard time believing that I have owned two cars plumbed the same way, possibly the wrong way. I have owned the 74 10 years and I doubt if those pipes were ever replaced.

Regards, Clifton
Clifton Gordon

Fair point. My Haynes only shows the earliest and latest plumbing, not the intermediate, and my Leyland Manual shows none of them. I would have expected the front circuit to come off the rear reservoir simply because it is nearest the pedal so there is less to go wrong, and the front circuit is the most important. But maybe they only discovered that after plumbing it the other way i.e. in the 'more logical' way of front to front and rear to rear.
Paul Hunt 2

Paul. The dual master cylinder (68-74 North American cars)is reversed 180 degrees from the later cars with a brake servo. The cylinder push rod is at the front and the pistons move toward the rear of the car when brakes are applied. The brake pedal is on the front end of the master cylinder.

I thought the 74 1/2 cars were supposed to have a brake servo, Tom said he has no servo so I wonder which master cylinder his car has?

Clifton
Clifton Gordon

The chamber that is closest to the pushrod from the brake pedal (normally also the end with the mounting bolts)is always for the front brakes. If the level in the reservoir is going down you have a leak somewhere. The master cylinder may be bypassing internally but that would not cause a loss of fluid.
John H

Clifton - that figures thinking about how the pedal works, which makes me wonder how it works on the later master with servo - is the piston pulled instead of pushed? Clausager indicates the combined master and servo was used from 74.5 i.e. rubber bumper cars as you say.
Paul Hunt 2

Paul, it is still pushed on the later cars with the servo. If you open the pedal box you will see that the pivot point for the pedal is higher and the rod to the master is connected below the pivot instead of above, so when the pedal is pressed the rod is still being pushed into the master cylinder. This style system has been used on US cars from the 50's and earlier.
John H

Paul the brake pedal has an extension above pivot pivot bushing. The push rod is connected to that extension as John mentioned. the attached image shows pedals from a 71 car.

Clifton


Clifton Gordon

I just checked the 70 MGB I care for, and the rear reservoir feeds the front calipers. It is a non-boosted system with a remote pressure failure valve.

hope this helps,
dave
Dave Braun

Looks like my car may be wrong. It works fine and it would be difficult to rebend the pipes to reverse the connections. I'm leaving it as is. Reversing it would use the secondary piston on the front brakes. I checked a Bendix brake manual that showed several master cylinders and it seems most cars feed the front brakes with the primary piston. I will be looking at brake pipes at some upcoming car shows.

Clifton
Clifton Gordon

John - thanks for that. Should've looked on Moss, that is exactly what it shows.
Paul Hunt 2

Thank you all for your responses. Owning a 74.5 that, I believe, has been "pieced" together with older and newer parts is somewhat confusing. This is what I have learned so far. I have rubber bumpers, dual circuit braking but no servo. My rear reservoir feeds the rear section of the brake pressure failure valve and then a single line to the rear brakes. Front to front to two front lines. I have been losing fluid in the rear reservoir but I have refilled, pumped repeatedly and can find no leaks while car is up on jackstands. (Nothing at the front calipers, rear wheel cylinders, brake lines, MC, unions or the brake pressure failure valve. I also am not seeing the fluid level go down now either so I will drive it a little to see if this shows the leak. Can someone confirm that there were some rubber bumpered 74.5s without servos. It wouldn't surprise me if this is not the original MC system. BTW, my MC rod extends towards the front of the car, is pushed back when foot pressure is applied and the pivot point is below the pushrod. I'll update when I know more.
Tom GIllett

I looked a 71 MGB today, owned by the same owner for over 30 years and the rear reservoir is connected to the rear section of the pressure failure valve.

The only Moss diagrams I have been able to find show pipe 18 (Pipe, master cylinder rear)going to the rear section of the pressure failure valve. The same information is in their on line catalog as I posted above and in the current Moss paper catalog.

I removed the cap from my reservoir yesterday and learned the divider for the reservoir is not in the center of the reservoir. It's offset toward the rear of the car, the rear reservoir measures 1 1/16" center to rear, the front reservoir measures 1 5/16" center to front. Clearly the front reservoir has a greater fluid capacity than the rear.

Clifton


Clifton Gordon

Tom,

I suppose if you had one you would know it by now, but are you sure you don't have a remote brake booster on that car? I just don't know how you could loose as much brake fluid as you mentioned and not see a leak unless it was going into a booster.

I do know what you mean about owning a Johnny Cash car. That can make things tough. As to your question about 74.5 cars without boosters, here is what Anders Ditlev Clausager says in his book The Original MGB:

"On North American cars, a standard-fit servo finally appeared in December 1974, at the start of the 1975 model year. This was the direct-acting type of servo which was combined with the master cylinder and the supply tank."

It seems, though it is not entirely clear to me, that the US MGBs did not have the remote boosters because the remote boosters that were used for other markets would not accommodate the dual line brake system that was required in the US. It seems that the US MGCs did, but they used two remote boosters. Also, anyone could have added an aftermarket remote booster to your car.

The rear compartment of the master on my dual brake 68 GT feeds the rear brakes, but as mentioned in earlier posts, the pistons move towards the rear of the car upon application. Mine is the one with the big (like 3 1/2 inch) round cap, and the rear reservoir is smaller than the front.

Best if luck,

Charley
C R Huff

I believe my system is the same as Clifton and CR describe although, accordig to both Moss and Victoria British, my serial number indicates I should have had a booster originally. My SN/VIN is 366777 and they say the boosters started at 360301. I definitely do not have any type of booster. Saying that, I feel my existing system should be fully functional if I can just figure out what is going on. I don't seem to be able to make it lose fluid while on the jackstands so I'll try to drive it some this week an look again for loss of fluid in the reservoir and under the car. I have to be careful because there are so many other fluids dripping under the car also! Thanks again for the updates and I'll repost as I find out more.
Tom Gillett

Tom



I have owned 2 74 MGB's One commisoned in
Feb 74 was a CB without a servo and the one I currently have a RB 74.5 MGB commissioned Nov 1974 does not have a servo either.
Unfortunately I am unable to sight the layout of the brake lines for you as the car is in the body shop under going an Omni flare conversion.
But it seem to me from memory that the front part of the reservoir feeds the front brakes. This makes sence to me as the reservoir is shaped in such a way as the fluid is allowed to flow over a baffle into the front part of the resevoir to ensure the front braking system, which does most of the braking, has enough fluid to do so.


Hope that helps
Bruce Mills

According to Clausager the direct-acting servo i.e. combined servo and master was only fitted to North American cars from the start of the 1975 model year, therefore the first rubber bumper cars from 1974 1/2 until then did not have them.
Paul Hunt 2

This thread was discussed between 09/03/2008 and 18/03/2008

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