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MGB Brake frustration

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You don't by chance have the rear wheel cylinders upside down do you? The bleed nipples on the wheel cylinders and the calipers need to be higher than the line feeding them. I don't even know if its possible to do so as I always check before install. Also yours wouldn't be the first MG car with something weird possessing it , which I guarantee was caused by someone working on it with just enough knowledge to mess it up. Like I said in an earlier post , sometimes when you replace everything : calipers, flexible lines , m/c, and wheel cylinders you'll fix a weird problem like this one without intending to . It only seems like it costs a lot but what it does is save a lot of time.
 
I believe there should be a plastic residual check valve and spring under the master cylinder rear circuit banjo fitting (end outlet). It retains a small amount of fluid pressure in the system to prevent air from being sucked in past the wheel cylinder washers as the cylinder retracts. If that is missing, inoperative, or reversed, you can bleed perfectly and get air in the wheel cylinders again.

New MC's sometimes don't come with those and the old one has to be reused. If you don't have one or can't find one, you can splice one inline. Available from vendors such as Speedway Motors or Summit Racing.
 
What years of MG's should have this valve ? I know there is no valve in the circuit as the car had already had the MC replaced before I bought it. When I put ANOTHER new MC in there was no such valve in either the old one or the new one !

OK, just went through the Bentley manual and there is one obscure mention of this/these valves. Under the master cylinder section Ma2 under "dismantling", item #28 states "Withdraw the springs and trap valves from the connection ports". No further description and no pictures. Are there trap valves on both the front and rear brake systems or only the rear ?

I have been diddling with this problem for three months. If any MG enthusiast has new or used trap valves and springs available I would be most appreciative. Thanks, Bob
 
The valve should be on all US spec dual master cylinders. Here's a poor pic that I found.
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They are sometimes called trap valves, sometimes check valves. If you look on the Moss brake parts picture they are items 21 & 22.
 

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The check valve is the little thing with the spring. It's inside the master cylinder. That larger metal piece is what screws into the master cylinder and you screw the hydraulic line into it. I'm sure that's not missing, or you wouldn't be able to attach the brake line, but the inside piece could still be defective or installed wrong.

That's a likely candidate for the cause. It's the only thing any of us have come up with, that explains everything that you're experiencing.
 
I'm keeping my fingers crossed but the fix has to wait a little while. I decided that it's such a pain in the a$$ taking the MC out that I would do the clutch MC at the same time.

IIRC there was a "trap valve" in the front brake circuit but none for the rear. The "new" MC came with the trap valve already installed but no where did it say to reuse/reinstall the old valve in the new MC. I guess I dropped the ball on that one.
 
I think I would consider adding an aftermarket residual valve inline at the rear of the car if you don't want to pull the master.

Get a Wilwood 10psi valve and splice it into the line just before the rear hose. Cost less than $20 plus some fittings.
 
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The new master cylinder really should have come with a new check valve. I'd be surprised if it didn't, but a defective one is always possible.
 
Sarastro said:
"the third or fourth iteration of "pump,pump,hold"..."

Hmmm. Not sure I understand this, but it might be the problem. When bleeding the brakes, you open the bleeder, push the pedal to the floor, close the bleeder, let up on the pedal. If you let up on the pedal without closing the bleeder, you can suck air (or fluid with bubbles in it) back into the system.
Click to expand...
What he's describing is a couple pumps of the pedal with the bleeder closed to build system pressure, then the pedal is held down, the bleeder opened, the bleeder closed, and the pedal lifted. It can be a helpful method for a system with smaller amounts of stubborn air. Using this procedure a large number of times at once has the potential to slightly aerate the fluid, however, and the car should sit for a period of time to let everything "settle" after bleeding.
 
I'll start off by saying the newest MC (#3) equipped with the trap valves did not solve the problem HOWEVER while doing some brake checking on my old drum brake Vet I may have stumbled on the real problem. On the Vet I have recently had a pull to the side when braking, The harder the brake application the more violent the pull. On investigation I found one front wheel cylinder with a little (very little) brake fluid inside the rubber cup on one end of the cylinder. Therefore that cylinder is not working at 100 % and the car is pulling toward the opposite side.

Today I checked the rear wheel cylinders on the MG. The last time I looked at them was a cursory visual check looking for obvious leaks of which there were none. Today I pried off the rubber boots and, yes, there is a little fluid showing inside the boot on the left rear cylinder. Since the LR is the line with the most air in it (every time it's been bled) perhaps this leak is just enough to allow a minuscule amount of air into the system every time the brakes are applied. I can only hope.
 
My prayers have been answered. The brakes are all better ! What an exercise in futility this has been. The last chapter of this story had me finding a little fluid inside the left rear wheel cylinder. You have to understand I'm a "if it ain't broke don't fix it" guy so contrary to my history I ordered TWO rear wheel cylinders.

Latest fiasco was breaking the pipe from the "T" on the axle to the left wheel cylinder. I was able to free the pipe from the WC but the fitting on the end of the pipe was seized to the pipe. Off to the parts store to buy a pipe nearly the correct length. On further examination I found the left WC was leaking on both ends and the excess brake fluid caused the linings on the brake shoes to delaminate ! Back to the parts store.

After replacing the left brake shoes and cylinder I decided since I already had another WC I might as well put it on the right side. Here I got lucky as the pipe came loose with no trauma. In hind sight it's a good thing i bought two WC's as the passenger side cylinder was completely frozen.

After a 25 mile jaunt around town the brakes are still firm and the pedal is steady , not sinking as in previously described threads. I can't believe I spent sooooo much time on a relatively simple fix but I have never before heard of a leaky WC inhaling air on the return stroke.
 
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First time for everything. Thanks for the heads up!
 
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