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MG MGB Technical - Brake problem

I'm passing on a problem as described to me from a club member. His brake pedal on his 68 BGT goes to the floor...very limited to no brakes. He replaced a leaking rear wheel cylinder, now sees no leaks in any lines or wheel cylinders. Rear brakes are adjusted up. When he bleeds the brakes, he gets fluid out of the rear bleeders and gets all the air out. However, he seems to get no fluid out of the front bleeders....has a helper pump the pedal several times and then hold down, crakes the front bleeder (either one) and gets no fluid. Both sides of the dual MC are full of fluid. Is this a bad MC? There is no leaking of the MC in the cockpit or under the hood. His pedal still goes to the floor. Thoughts?
R Dougherty

Hi there,

Sounds like the MC is shot. I would crack open the lines on the MC that feed the two front brake hoses. And pump the pedal Very slow to see if fluid comes out. You should see lots of fluid come out make sure you have rags around so You can clean up the wasted brake fluid. If fluid comes out with pressure there is a clog in the brake hoses or lines.

James D

Never had a dual circuit B, but dont they do one front one rear on each circuit?
Stan Best


Others have said the same thing to me. However, when I bleed my rear brakes, both sides lower the level of the rear compartment of the brake fluid bowl. Also, there is only one line going to the rear of the car, and it splits to the two rear wheels at the rear axle.

When I bleed the fronts, both sides lower the level of the front section of the fluid bowl.

Seems to me that clearly means on a 73B roadster at any rate, it's the rears on one circuit and the fronts on the other.

BH Davis

I think it's like that on all of them, but Haynes shows the rear brakes coming off the front port. I believe cars designed after the B had diagonal circuits, then others with twin front calipers.
Paul Hunt 2


No, one circuit is for the front and one circuit is for the rear. That's how it is on most of today's cars also.
The only exception I have come accross is Volvo brakes. On Volvos one circuit is both fronts and one rear and the other circuit is both fronts and the other rear. There are two bleeder screws on each front caliper and two flex lines going to each front caliper.

I had wondered about this, clearly still having brakes is no use if the diffential braking effort on the steered wheels causes you to spin into oncoming traffic. So yes, this means it's the master cylinder at fault here as well as the rear cylinder, so both ends out at the same time.
Stan Best

The 68-75 North American Specification cars has a dual master cylinder with a single line coming from the front, and a single line coming from the rear of the master cylinder bore. The lines go forwards into a five way connector having the brake system warning light attached. A single line runs from the connector to the rear brakes and two lines run to the front brakes, one for each caliper.

Yes, this can be a master cylinder problem. But, it can also be a problem at the four way connector, which is much easier to gain access to. Jame's comment about unloosening a brake line, while the pedal is depressed, is a good one. If there is fluid under pressure, the master cylinder would seem to be working. If not, the master cylinder may be bad.

My copy of the Factory Workshop manual is the Robert Bentley reprint. On the subject of bleeding the brakes, it references the procedure used for the earlier, single piston, master cylinder. It does not address whether the brake failure warning light has to be unscrewed from the five way connector for the system to be bleed properly. Later master cylinders do require that the brake failure warning light switch be unscrewed several turns for the system to be properly bled.

I wonder if Bob's friend still has the switch in place and what effect loosening it would have in allowing fluid to flow to the front brakes.

Les Bengtson

"one circuit is for the front and one circuit is for the rear. That's how it is on most of today's cars also"

That is how the MGB is, but if the front circuit fails it means you have virtually no braking. Which is why diagonal and dual circuit front calipers are used now. MG ZR, S and T use diagonal braking, FWIW, one hopes that it isn't so imbalanced that it flings you off the road or even worse into oncoming traffic. This is deja vu, there was a similar discussion a while ago about ABS and what effect that has on directional stability. The effect of having the front wheels on surfaces with very different coefficient of friction is much the same.
Paul Hunt 2


Is the "five way connector" a proportioning valve?

Bruce Mills

There never has been a proportioning valve on an MGB, front/rear balance is controlled by the relative internal dimensions of calipers and slaves, which is why 4-cylinder GTs got larger slave cylinders than the roadster.

On single circuit systems the five way connector is where the inlet, two front pipes, one rear pipe, and the brake light switch connected. On early dual circuit systems it was where the four pipes and the balance switch connected.
Paul Hunt 2

Has the car a servo? The take off for the two front brakes is a notorious air lock. The factory vacuum bled them to get round the problem. Also check the orientation of the air valve. It's better at 4 or 8 o'clock as this makes bleeding easier. Check also the condition of the flexibles, when they deteriorate they block up, but both sides at once is a bit of a coincidence, but not impossible!
Allan Reeling

Thanks for the comments! I"ll see if he has pressure at the front, if he doesn't...bad MC. If he does, cloged line or collapsed flexible brake line. I'll check if he has the brake failure warning light switch and see what happens when it is loosened. The car has no servo.
He is new to the MG, having won a "restored" 69 MGBGT in a raffle. The car is nice looking, but he's had a lot of little problems (overheating, stalling etc.)He's getting an education on MG's the hard way!!
R Dougherty

OK, now I have a few questions! I went out to the garage to take a look at my 69 roadster so I could visualize what I needed to tell him to do.....which section of the dual MC is for the front brakes? It would appear it is the front section but I want to make sure.
Also, the 4 way union with the the switch supposed to he hooked up to something?! I've never paid much attention to the union (if it ain't broke...) other then to see if any fluid was leaking. Mine has nothing connected to the plastic switch, is this right? I also checked the archives and others talk about opening the plastic switch a couple of turns when they bleed their brakes...I've never seen this in any this something I need to do? I've never done it before and my brakes seem to bleed ok....
R Dougherty

Bob, The front section of the reservoir toward the front of the car is for the front brakes.

The plastic switch on the brake failure valve does not need to be turned out to bleed the brakes (that information applies to rubber bumper cars with a Pressure Differential Warning Actuator installed in the cylinder). Your car originally had two wires hooked to the plastic switch connecting the switch to the brake failure light on the dash. It's warning device only and does not have any effect on brake operation. I don't think your friend will find the brake failure valve to be the problem. The shuttle can shift when one circuit has lower pressure than the other but it does not block brake fluid flow. See the image for a typical brake pressure failure valve.


Clifton Gordon

Thanks Clifton, that answers my questions. My brake failure light inside the cockpit always lights when I press to test, I thought this meant the system was checking good. Now I realize it is only testing the bulb in the dash switch as the plastic switch under the bonnet isn't even hooked up!! Guess I'll try to find the correct wire and hook it up.
R Dougherty

According to the Haynes Automotive Repair Manual from the section on "Bleeding the hydraulic system (Pressure Differential Warning Actuator in circuit)", "before commencing to bleed the system, take the leads loose from the Pressure Failure Switch and unscrew it 3 1/2 turns to bring the plunger clear of the piston. Bleed the front caliper nearest to master cylinder first, followed by the other front caliper and then the rear brakes. After checking for a firm pedal, tighten the pressure failure switch and reconnect the wiring". I did not read this section because I had performed brake bleeding on many American cars for over 6 years and did not think I needed to, initially anyway, but found out differntly.

I had a 1969 MGB-GT built for the USA market (the first of three GTs I would eventually own and later the other 11 british cars I maintained/fixed/rebuilt over the years). I had to rebuild and bleed both the brake and clutch master cylinders on each of 14 total british cars. Based on my similar problem solving experience with the brake system, there are two potential causes of your problem, (if you have a tandem brake master cylinder with integral Pressure Warning Actuator on your car).

The first is that you possibly did not follow exactly the correct procedure mentioned above to bleed the brakes. This caused me the same problem the first time I rebuilt the brake master cylinder. I did not follow exactly the correct procedure and experienced the same problem with the front brake bleed not producing fluid nor a firm brake pedal.

On my second try I referred to the Haynes manual and followed the recommended procedure exactly which resulted in producing fluid but only had a firm pedal initially when pushed then went to the floor slowly. Progress but not resolved.

I decided to take the master cylinder apart again and inspect everthing, especially the piston, cylinder, and seals. Fortunately I had ordered two of the rebuild kits since this was my first british car master cylinder rebuild and I needed to make sure that I could get my car back on the road by Monday to get to work.

What I found was a less than perfect seal cup by testing for bleed by, (the cylinder looked to be in good shape and surface). After replacing the seal cup and retesting for bleed by and getting none I put it all back together, reinstalled, and followed the exact procedure for bleeding. Success!

Just in case you did not know, you must ensure on any rebuild of any type of master cylinder, that you must must, not only closely inspect all new parts, but also ensure you have an extremely clean (almost sterile)environment including your hands throughout the rebuild procedure, whenever you handle any internal hydraulic parts. One grain of very small sand on a seal can cause damage to the seal and/or metal cylinder and cause bleed by that could cause an immediate failure or eventual failure prematurely further down the road.

I still remember this first experience of rebuilding a brake master cylinder as one of my best learning experiences since it taught me to always read all the information, think about it completely (since I have yet to find a single manual that covers everything 100% that you may need to know), and follow the proven procedures!

Hope this helps you and possibly others too.
M Baxter

This thread was discussed between 08/09/2007 and 16/09/2007

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