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MG MGA - Replacing master cylinder seals in situ?

Hi everyone,

I have just returned my MGA 1600 to the road after a fairly lengthy layup (12 months+).

However after only a couple of miles down the road both front calipers locked up. Reading through the archives it would appear that the brake piston in the master cylinder is pressurising when the brake fluid (non silicone) warms up - presumably the result of seals which have swelled or deteriorated with non-use.

Prior to the lay-up the entire braking system was completely overhauled (including stainless steel lining of the master cylinder) and has less than 1000 trouble free miles under its belt.

My question is, can I simply drain the master cylinder (either bleed dry or syphon out the fluid) and replace the master cylinder seals in situ? The reason is I prefer not to disturb the banjo seals - a job I don't have fond memories of!

NB. This is a RHD car so access to the master cylinder is pretty good (pic attached).




Steven, before you dismantle anything check that there is clearance between the piston rod and the piston. If the piston can't return all the way back you will get pressure build up in the system and the brakes won't release because the fluid can't return to the master cylinder. Anyhow such a petty little car should not be acting up !
S Sherry

Steven, it is possible to do in situ, unless you have back trouble! Watch out for that non silicon fluid on your nice shiny paint though.

In addition to the comments above, another (less likely) possibility is there could be a restriction (old piece of rubber, etc.) in the brake pipe preventing fluid from returning after applying the brakes.

Neil McGurk

SH I'm told if you you drain the reservoir its possible to insert a thin piece of fuse wire down the fluid pressure release hole to check that its clear of the rubber seal. The wire has to be very thin and perfectly straight - if you kink it or the end drops into the cylinder then you've got problems.
J H Cole

Nice car!! The non silicone brake fluid will definitely and slowly eat any paint that it contacts. Wash well even if you don't think any was spilled.
Russ Carnes

Also check that the front brake hoses are not so aged that they're collapsing and holding pressure when the brake pedal is released. This happened to my MkII when the hoses were about 20 years old.
Steve K

I replaced mine in situ a couple of years ago. But the piston became stuck in the cylinder and I couldn't shift it without removing the master cylinder - and creating a lot of mess. It would have been easier to remove it in the first place - that way you can work the piston with the new rubbers a few times to make sure it returns easily against the spring.
Cam Cunningham

Anyone know who is making the best seal sets now : I need to do the clutch side of mine again, last time I had to reuse one of the old seals because the new one caused the piston to seize. Now that seal seems to be giving up the ghost.
dominic clancy

Hi Dominic - that's exactly what happened with mine - glad it wasn't just me being cack handed as usual!!
Cam Cunningham

Thanks guys,I think I will go the safer route and take the m/c out of the car for a full diagnosis.

Will report back what I find.



This thread was discussed between 19/08/2008 and 28/08/2008

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