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MG MGB Technical - Alarming loss of brake pressure

Hi Guys,

This is by the way of being somewhat urgent:

On the way to work today I suufered an alarming loss of brake pressure - i.e the pedal went to the floor with only marginal braking. On pumping the pedal I could achieve suffucent braking to stop the car.

I haven't looked, as I had to go straight to work, but could you give me some ideas as to what to look for and some possible remedies?

Thanks in advance,

R Barrell

Rich, In order of likeliness, if there's no loss of fluid from the m/c then it's the m/c seals; they let fluid by as you press the pedal which then stays in the reservoir. Rebuild m/c with new seals, if the metalwork is good, or replace.

If the fluid level's down you either have a leak somewhere or, if everything's dry and you have a servo, the servo seals may have failed and drawn the fluid in.


Could be as Rich says but the common cause of this is simple brake fade where the fluid just behind the caliper pistons boils and the pedal then sinks. I assume that you are using conventional brake fluid which is hydroscopic and cotaminates after a couple of years and should be changed.
Iain MacKintosh

Thanks guys - I can't vouch for the brake fluid - I've had the car for a year and *I* haven't changed it. I can't say when it was last done.

I'll try a fluid change first before stripping the master cylinder. It will take a while for the parts to arrive from Uncle Moss anyhoo.

I must say I am not looking forward to the drive home... :(

Thanks again,

R Barrell

Before you set off Rich try the pedal for pressure. If it holds up then it may be fade as Iain says. If it slowly sinks with cold brakes it will be m/c seals.


Yes that's spot on and if it does hold pressure then a simple change of fluid will save you having to rebuild the m/c.
Iain MacKintosh

Hi Rich

My brake pedal went to the floor when I first took my car out of winter storage. I drove home carefully with the handbrake using back roads. In my case the problem turned out to be a leaking right rear brake cylinder - no doubt the seal had dried out during the 4/5 months without any use. Ironically I had changed the left cylinder the previous year but held off doing the second one due tothe difficulty getting the E-clip back. THis year I used a 5/8 circlip!

Good luck.

B.J. Quartermaine

The 5/8" circlip I have to agree makes the job a whole heck of a lot easier. I couldn't get any of the tricks of the trade to work, but never bought the special tool either.

Best regards,
DT Toms

Same thing just happened to me 2 weeks ago. One of the front brake hoses ruptured. Quick fix, and back on the road within an hour or two.


How old are your hoses? If you don't know they need replacing NOW!

I found out the hard way about a month ago when a panic stop resulted in a pin hole on one front hose. Lucky it didn't simply burst...
Richard Smith

Hi All,

Thanks for your help, I just though I'd update:

When jacking up the rear of the car I noticed that the near-side rear drum was dripping and sure enough the seals on the brake cylinder were deformed and leaking. I also checked the pistons and they looked scored (I don't know if this is normal). Anyhow, I've just buzzed off a letter to Dear Uncle Moss, and am waiting patiently to receive the replacement parts.

I had to get the train to work this morning, so I can't wait to get the GT back on the road!

Thanks again,

R Barrell

Don't forget you will almost certainly need to replace the brake shoes as well, brake fluid contamination ruins the lining material.

From my experience, it often goes that if one brake cylinder fails, the one on the other side wont be far behind it. I'd take the drum off and check that one too if I were you - you don't want to be replacing the brake shoes twice.

Mike M

Hi Rich, Good you've found it without any other upset. If the shoes have good linings you can often clean them OK with degreaser or aerosol brake cleaner.

To save time whilst waiting for Moss I'd start disassembly to see if any other parts give problem. If you put a clean piece of polythene under the master cyl cap and screw it down tight it will stop fluid running out the back pipe. You can then take off the pipe to see if it comes away easily or is frozen and will need replacing.

Then clean everything up ready for the new wheel cyl. After installing keep an eye on that cyl btw as the flare on the pipe doesn't always seal to a new cyl and may need quite a bit of tightening. The bleed screws on 3 pattern ones I've used also needed quite a lot of tightening to stop a persistant drip.


This thread was discussed between 24/06/2005 and 27/06/2005

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