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MG MGB Technical - Brakes staying on after pedal release

Hi, My refurbished MGB GT 1800 has a brake problem - after braking at junctions/roundabouts etc, the brakes remain on for about 1 - 2 seconds after releasing brake pedal, which feels very strange especially if I need to pull away quickly. I have a new master cylinder and seal replacement of the servo. Brakes seem to be fine otherwise, is this something to be concerned about? Appreciate your thoughts. Pete
P J Dyble

are you new to MGBs or been away from them for a long time

if you want to you could include your name in your usermame and put up your vehicle profile for others to see with a photo as it could help as there are many variations other the years

first things that come to my mind are -
are the flexi-hoses new
was the brake fluid fully flushed out and renewed
if it's both brakes then possibly servo related or MC or its seals or pushrod or hydralic or flexi-hoses or combination of all
if it's only one side at front then possibly caliper but that would give pull to one side

never assume new parts can't be faulty
Nigel Atkins

Pete - as Nigel says, flexi-hoses are a prime suspect. They need a good looking at if you haven't already done so. They may look ok on the outside, but will disintegrate internally and eventually cause a blockage (this happened on mine recently). You may be at the stage where one or more of your flexis are almost blocked, but still allow fluid through under high pressure when the brake pedal is operated, but when the pedal is released the fluid returns under lower pressure and takes a few seconds to flow back through the restricted flexi.
It may be worth putting the car on axle stands, operating the brake pedal, and checking which wheel(s) have 'sticky' brakes.
Brian Shaw

My guess is the seal replacement in the servo - I had had exactly the same in the past and no longer fit seal kits to servos, only new servos.

Take out the air valve piston and ensure it moves freely up and down its bore - if it is sticking it will give delayed release.
Chris at Octarine Services

This is a not uncommon occurrence with the remote servo, and is caused by the air valve sticking (as Chris says). Some say the servo should be mounted with the air valve at the lowest point as per the after-market kit fitting instructions, others say to lubricate the piston of the air valve with brake fluid or silicone grease, yet more say to polish the air valve piston and bore with fine wet and dry. My roadster has been doing it in hot weather for some time, usually a tap on the brake pedal releases it. A pal's was so bad we had to disconnect the vacuum hose from the servo and plug it with a spare spark plug half way round a tour of Ireland. Neither have had replacement seals.

That causes all four brakes to stick on, i.e. balanced braking. If a front hose is delaminating and acting as a one-way valve that will only affect one front wheel, so should pull to that side. The rear hose will only affect the rear brakes, and will barely be noticeable, apart from hot drums.
Paul Hunt

as suggested on another thread on here - if the servo is faulty rather than replacing it with an expensive good quality or cheaper poor quality servo you could remove the servo and not replace it
Nigel Atkins

Hi, thank you for all our replys - interesting! Appology for late reply. Did not ever consider hoses. They seem to brake evenly all round so assume servo first point of call - question, comments seem to suggest that if I disconnect vacuum hose the brake system will perform close to normal with servo? Will be checking all points this weekend.
Nigel, my name is Pete and I am new to forum/site so still finding my way around. I have completed my vehicle detail section and uploaded photo - hope you can access them?
Thanks Guys nd will keep you informed of progress.
P J Dyble

If you disconnect the vacuum hose, you must plug it to prevent excess air being drawn into the inlet manifold. This will prevent the servo from operating, but the fluid will still be going round the servo. To permanently run without the servo you must remove it completely and use a new piece of pipe to connect the brake master to the 4-way union. If the servo is there, the MOT inspector will insist it is working. As has been said on here many times, the brakes are fine without a servo. After all, between 1962 and about 1972 the cars were made without servos. When the servo was fitted the brake components weren't altered in any way, so taking the servo off your late model car will give you a slightly firmer pedal, but the brakes will be just fine.
Mike Howlett

Servos were fitted as STANDARD on V8s from the outset and 4 pots from August 1973.

The MOT will be failed if a servo fitted as standard is missing.
Chris at Octarine Services

Don't agree Chris. A few years ago when I was having my servo replated I put my B in for Mot. Having previously discussed the matter with the tester he said that if the cars were built with or without servo he would accept it with the servo removed and a connecting pipe fitted. Passed no problem.
Iain MacKintosh

He is wrong - see the MOT manual-

C. Servos and Master Cylinders

Servos and Master Cylinders
Check servos for:


condition of vacuum pipe


Where an adjustment indicating rod is provided, check if brake adjustment is necessary.

Examine the condition of chassis or body structure and panelling around the master cylinder or combined master cylinder / servo mounting.

Reasons for failure -

A servo:
missing when fitted as standard

vacuum pipe deteriorated, kinked, split, collapsed or excessively chafed


Adjustment indicating rod shows brake adjustment is necessary.

Deliberate modification which significantly reduces the original strength, excessive corrosion, severe distortion, a fracture or an inadequate repair of a load bearing member or its supporting structure or supporting panelling within 30cm of the master cylinder/servo mounting, that is, within a ‘prescribed area’, see Appendix C.
Chris at Octarine Services

Hi Pete,
well your user name hasn’t changed yet and your vehicle profile isn’t on public view

to change your user details and add your vehicle profile to public view –
. click on ‘Customise’ (at top of page)
. then on the page you are taken to you can alter your ‘Username’ and ‘Location’ and whatever else you want
. at the bottom of the page click ‘Yes’ to ‘Display a link to a summary of my vehicles with my posts’ and note (if you set to Yes, you still have to mark each vehicle as viewable)‘
. finally click on ‘Save options’ and you’re done

as far as the servo disconnection goes no one would want you breaking the law so if your car is post ’73 then only disconnect the vacuum hose and plug it if you can test the car off the road
Nigel Atkins

Hi Pete

Your vehicle profile is now on view and it's nice to see that 'RXC' is a Solihull registered B.

*XC was originally a London designation, but from 1963/4 until 1974 it was assigned to Solihull.
Dave O'Neill2

Hi Pete,
yes the 'View vehicle profile' is showing to me now too

I think maroon is understating the colour and your car is a bit more than standard spec, I've no idea what the wheels are but I bet others will know exactly

good news as it's a pre-'73 you can do the disconnect and plug servo test and even remove the servo if you wanted

IF you removed the servo the braking of the car remains the same just that you might press the pedal harder which you might like or might not, you'd certainly be able to get used to if you wanted

slightly off-topic but related - always remember that good quality tyres play a big part in braking too (and steering, handling and ride comfort and noise), tyres that are hard through age and/or lack of use regardless of tread depth left will be far from reasonable - (see down this page) -


Nigel Atkins

Point taken Chris, but if the servo was removed and the pipework made good how is the tester ever to know whether the servo was there in the first place or not??
Iain MacKintosh

depends whether he knows that B's had servos - mine would but then he does about 20 MGBs every spring for me!
Chris at Octarine Services

This thread was discussed between 20/06/2013 and 05/07/2013

MG MGB Technical index

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