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MG MGB Technical - Brake Pedal Travel Varies

My 73 MGB starts the day with a very firm brake pedal. As the day goes on, the travel can get longer and longer. Pumping makes no difference. The car still stops OK though. It recovers over- night back to normal.

Yesterday we were heading into a couple of special tests on Kemble airfield and the travel was quite long – it looked like this might be quite interesting……

As I queued for the for the second test I noted the the pedal had come back up – rock solid and stayed like that for the test which involved a lot of hard braking. It was Ok for the 60-mile journey home as well

I do not have a servo having removed it earlier this year, so it’s not that. I have Standard Dot 4 fluid and Mintex 1144 pads

Any ideas?
Mike Dixon

I take it there's no change in fluid level?
Dave O'Neill 2

If it only happened when using the brakes hard, and not in gentle driving, then I would be thinking of moisture in the fluid boiling under heavy braking. Was it replaced when you removed the servo? Was it a newly purchased can?

If it was OK for the second test with lots of heavy braking, and a 'normal' drive home, then I am coming down more on mechanical somewhere and not hydraulic.

Pumping not making it harder and higher tends to eliminate air in the system, but how that would get out overnight would be a real mystery.

I take it the pedal doesn't sink under sustained heavy pressure?

If the pedal resistance is the same when it is longer then I'd be looking at brake material travel.

Pads seems unlikely, possibly shoes although how it reduces overnight would equally be a mystery.

Does pulling the handbrake up a notch or two make a difference when it is long? Can you feel a change in the pedal position when that is partially applying pressure and you then pull up the handbrake?

A possibility is the brake master pressure seal intermittently not seating correctly. I had similar with a clutch master where the spreader had cracked so the seal was cocked in the bore - sometimes OK but sometimes the clutch would engage slowly with the pedal held down. The cracked one is lower in the first attached and a good one upper. These are not available AFAIK, I had a 'spare' from a replaced master. But they are only used with the cup-type seal not the ring-type, and you can't reliably tell which you have without taking the piston out.

Note the brake master has a slow-return valve in the end of the spring at the bottom of the cylinder (white plastic bit in the second picture), this allows you to 'pump up' the brakes if you have air in the system.






paulh4

I will try the different tests when it next happens but my suspicion is it might be the master cylinder form what you say

Anyone used the 'Classic Gold' or similar after market ones? They seem a lot cheaper.
Mike Dixon

Most likely master cylinder piston not fully returning at times/sticking - or could be excessive freeplay in the front wheel bearings knocking the pads back, with that you'll have a decent pedal stationary but as soon as you go round some corners the pads get knocked back and a long pedal is the result--
I'd be checking for excessive front wheel play first, then if that's ok move on to the m/cyl overhaul or replacement.
William Revit

Willy

I thought about wheel bearings, but Mike said that pumping makes no difference.

I think you’re right with the master cylinder piston sticking.
Dave O'Neill 2

I thought about brake knock back (well it was suggested to me in the bar after the event!) but as it happens in normal driving and also does not rectify itself when you apply the brakes, which I think it does if that is the problem?

I will have a quick look at wheel bearing play but I think it is a new master cylinder

Mike Dixon

It would certainly be worth dismantling the MC to see if there's an obvious issue with it.
Dave O'Neill 2

If the master piston doesn't fully return then it can't clear the bypass port to the reservoir so you end up with the same amount of fluid in the lines etc. as before. In that situation heavy braking will cause the fluid to expand and tend to cause the brakes to bind when they should be released, resulting in _less_ pedal travel than normal.

I get this when my roadster servo sticks in hot weather - the brakes drag and pedal travel is very short and hard. I realise you have removed the servo.
paulh4

This thread was discussed between 07/07/2025 and 09/07/2025

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