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MG MGB Technical - brake bleeding woes...

I know - another brake bleeding question. Here goes. I just rebuilt my ENTIRE brake system with new parts from Moss including cupro-nickel tubing and stainless hoses. I also converted to new GT wheel cylinders (no, not the cheap ones) in my roadster. Its a '71 with no servo, and the pressure failure switch removed.
I bled the system from the bleeders with a power bleeding system that I made using a Mity-vac bleeder cup (reservoir) and an electric 7cfm vacuum pump used for pulling a vac on air condition systems. I bled the entire volume through the system 3 times and when finished, it still takes 3 pumps of the brake pedal to get the pressure up. The rear brakes are definately adjusted properly. Where is the air hiding?
Jeff Schlemmer

Jeff - Did you bleed the M/C independently first? This is real easy to do. Just run the outlets back into the reservoir with some tubing and then pump the cylinder manually until all bubbles are gone. Good luck - Dave
David DuBois


I had this problem once. It had to do with the proportioning valve front/rear being stuck in one position or the other; hence all bleeding was only going to one side of the system. (that was my theory after the fact, never proven) I solved it by reverse bleeding the whole system (pumped fluid in FROM the bleed nipples). I had to make a mod to a nipple (ground the end off a spare) so no air entered as it was pumped in. Took me about a gallon of brake fluid to figure out!

Another possibility is that your bleed system is too agressive. I have recently found out that the furious pumpimg we all do is wrong; it should be a very slow pedal movement. This is even more critical with the silicon fluids. The last time I did my 67 I let gravity do ALL the work. Best pedal ever. Too bad it did not work for the clutch.


David, this is the first master cylinder that I did not bench bleed. Due to the poor positioning of the MC, it's very difficult to do without spilling fluid everywhere. I've read in the archives it's not necessary on these cars? I should have tried anyway!

Pete, thanks for the ideas. I did center the piston in the proportioning valve, but maybe the bleeding moved it? I'll pop the bolt out to check!
My bleeding system is quite aggressive, but it can be controlled with a valve on the pump. Before I went to it as a last resort, I was pulling fluid through with the MityVac hand pump. I thought the big pump might force through any trapped air? Guess not!
Thanks again guys!
Jeff Schlemmer


I put in the same system you did with all new master cylinder, calapers and new pistons. I did not have to bench bleed the master cylinder, just left all brakes open for about 2 hours with the master cylinder full. Did wonderful. Then I started to bleed the system.

Bleeding the system will shift the proportioning valve. If your gaskets are good, you can bleed the system with the cover off of the valve. I used a small dull probe to center the valve and left it in place while bleeding. My lovely assistant couldn't understand why she had to hold it in place so long. But having my wife hold it in place assurred that I got the presure to each wheel. Then when I was finished, I was able to close it back-up and test to ensure the valve was in place.

After closing the system up, I checked each wheel individually for full function. Very hard pedal, very little movement before the calipers and cylinders start their action.

Cris DeYoung

I had the same problem once on a '67 B. I eventually found my "rebuild" of the front calipers had some issue. I discovered the problem by clamping the rubber brake hoses to close them (gently, using vise grips and shop rags). I found that when I closed off the front lines I had great brakes.
You might try a similar test to elimenate areas of the brake system.
David Steverson

Pete. Where were you two weeks ago. I think the "too aggressive" process may be the issue. I was trying to bleed my brakes with the mity-vac, and after two pints of fluid i gave up. The next day, i had my buddy come over and do it gently but manually. Finished in no time at all. I tried the gravity method, but it didn't fo anything. Is there a trick to that?

70 B
Ken Harris

I have found that the old 2 person method as described in the Haynes manual, slowly pushing the pedal down always works perfectly. Aside from hearing complaints of boredom from the pedal pushers (I've gone through all my kids and my wife)it works great :>)



Try these and you can do the same thing without the dependence of local "slave labor" ;-)

I have them on my A and they do work.


Larry Hallanger

Just an update. The piston is only slightly off center in the proportioning vavle, which tells me its giving slightly more bias to the front - which is correct. The good news - my wife agreed to buy me "speed bleeders" so she doesn't have to help bleeding the brakes! Since there is still about 2" of mush in the pedal, I'll try bleeding it manually.
Thanks everyone!!!
Jeff Schlemmer


I think the gravity thing only works on early cars. I discovered it be accident; good thing I was using silicone fluid or my new panasports would have neeeded a re-paint.


Maybe I'm just lucky. But I've been able to bleed the brakes the manual way without an assistant. ('74 dual line system, no servo.)

As the ancient manuals say, connect a hose to the bleeder and put the free end in a jar of fluid. Crack the bleeder open a hair. Get in the car, and gently press the pedal. Let it up gently as well. I take 2-3 seconds to get the pedal through it's stroke (each way).

Must be the weight of the fluid in the system against gravity that keeps it from running back up the tube. And perhaps the valve in my master cylinder is more forgiving than others. But for whatever reason, it does seem to work for me. And it may be worth a try for others who get in the middle of a brake job and find out all assistants have migrated to the mall for the day.

Matt Kulka

Please see my post under the "Brake fluid reservoir SUPPOSED to open to air?".

The gravity method works great on my '73. Don't know about later cars though.
Richard Smith

Never bench bled, but I have always had to employ an extra step in bleeding, even when the master hasn't been touched, which makes me think bench bleeding wouldn't have helped me. This is on both a 4-cylinder and a factory V8. After bleeding either with the foot pedal or a gunsons EeziBleed until no more air bubbles, I always get the long pedal that pumps up, that needs pumping up again a few seconds after being released. I get an assistant to stand on the foot pedal really hard while I rapidly open and close each caliper nipple. This *always* blasts an extra 'lump' of air out, and after that the pedal is fine.
Paul Hunt

Matt, that's the one thing I never did - put the hose in a jar of fluid. It only makes sense. That would prevent air from re-entering the lines. Don't I feel stupid.
Paul - I'm glad to hear someone else has the same problems as me. I've never had to re-bleed the brakes on american cars. I always did like a challenge.
Jeff Schlemmer

Just installed the "Speedbleeders" and they really worked! Within 15 minutes the problem was solved. It was air trapped in the rear lines!
Jeff Schlemmer

This thread was discussed between 03/04/2005 and 12/04/2005

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