MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Variable Brake Pressure - Challenging Problem

I have a challenging problem that hopefully, collective wisdom can help me solve! I’ve owned a number of MGB’s, I do all my own work, race a B GT in vintage racing and I consider myself a reasonable shadetree mechanic but this one has me stumped.

I have a ’70 B GT that I drive nearly every day. It has servo assisted brakes that appear factory fitted. The engine is a series 18GG and was slightly tweaked when I bought it. The woman I bought the car from didn’t know what had been done but I can confirm that it has a ported and polished head, 1 ¾” SU’s and I suspect it has a mild cam. The car has a 5 speed gearbox. It came with receipts for a complete brake overhaul including new servo and brake master. In normal driving the brakes are very firm and solid with maybe ¼” free play. It stops straight and has some of the better MGB brakes that I have experienced.

The problem comes with high rpm’s. When I drive the car at high revs, irrespective of gear, and hit the brakes, sometimes the pedal goes nearly to the floor before stopping. After driving the car at lower rpm’s, the brakes return to normal. This also occurs after climbing a mountain (not high rpm's but low vacuum for an extended time).

The brakes have been bled and re-bled and I am confident there is no air in the system. I replaced both the master and servo and the one-way valve for the servo on the manifold, all of which made no difference. I installed a vacuum gauge in the line from the engine to the servo – the readings seem low to me: cruising @ about 2500rpm/5th gear – 8-9 in., idle – 11-12in., hard acceleration – 2-3in. (as to be expected). Pads are thick and even, rear brakes are thick and correctly adjusted, there are no signs of leaking fluid, compression is good, timing is set correctly, the car runs extremely strong.

My theory is that because of engine modifications and larger carbs the engine produces fairly low vacuum, which is too low for proper brake servo functioning at anything over moderate engine speeds. However, the thing that I find confusing is that I always thought servo assisted brakes, without vacuum assistance would require much greater pedal pressure but would not affect pedal travel – in effect – exactly the opposite of what I am experiencing.

I could convert over to a non-servo braking system but before I go to that much trouble I'd like to hear other opinions.

Bob
Bob Lasater

This sounds like over servo. The check that your servo is working is to put your foor on the brakes at astandstill, then start the engine and you should feel the pedal go down as the servo kicks in.What happens if you clamp the feed from the manifold to the servo?
Stan Best

I would suspect the non return valve (check valve) in the vacuum line from the manifold. It holds the residual vacuum and if it fails the booster returns to manifold pressure under open throttle positions.
Sandy
ss sanders

To me is sounds like the pads are getting pushed back, as that will cause the extra pedal travel
If it were vac. related you would have either a hard pedal with low vac or very touchy brakes with excessive vac.
If it's ok round town vac. isn't an issue
You probably have one or more of the following

Loose front wheel bearings
Distorted rotors pushing the pads back
Or maybee hard caliper seals pulling the pistons back a bit but you would think that would be more of a time thing than a revs thing

You could test to see if it is vac. related by disconnecting the vac supply and going for a drive and see if you get the same result - or not

Strange one, look forward to hearing what you find-
Willy


Could be a rusted up sticky piston or shaft in the booster I guess but if it's had a new one probably not
William Revit

American spec cars got dual-circuit brakes in 1968, and there wasn't a servo for those (integral with the master) until the start of the 75 model year. So either you have a retro-fitted later dual-circuit servo master, or a UK spec single-line system with remote servo, or a non-standard modification.

At first sight it does seem that the only thing that can cause this is the pads getting pushed back, as servo problems will only cause a high hard pedal if it is not working. It would have to be not working most of the time, and only intermittently working after high-speed running, to get an apparent 'problem' of a longer pedal. However some dual-circuit systems (I thought only the non-servo ones) had residual pressure valves in the outlets of the master, and if these fail and let all the pressure in the lines back into the master, the effect can be a very long pedal.

The easiest way to eliminate the servo is simply disconnect the vacuum hose from the servo and plug it, and try low and high speed running. However be aware that whilst the remote servo gives very little assistance (it was optional), and you would hardly notice the difference, the later factory integral servo gives significant assistance and the brakes will need a much harder push.
PaulH Solihull

Update:
Pressed the brake pedal before starting car - started car and brake pedal went down slightly - as it is supposed to.

Clamped off the servo vacuum line and went for drive - hard, firm brake pedal, even and consistent. Actually seemed to require only very slightly more pressure than with the servo.

Jacked up front, removed wheels - right front hub had way too much end float - really quite excessive. I think I've found the problem! End float pushing the pads back into the calipers.

I have a fresh set of wheel bearings so I'll clean up everything up and put new bearings in both sides while I have it stripped down.

Thanks for the comments.

Bob
Bob Lasater

Thanks for posting your results. If the brakes behaved well without the servo then my money is on a servo problem, the pads would still get knocked back and need to be pushed in before working servo or no servo. You cant do any harm with fettling the wheel bearings and I hope it fixes the problem, look forward to further results.
Stan Best

This thread was discussed between 10/02/2011 and 12/02/2011

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now