Suspect Brake Master Cylinder..?

[Graham Ness]

My Mk3 Spitfire recently passed its MOT with no advisories, so I was feeling somewhat proud.  But as has been said before, pride comes before a fall.

 

Since the MOT I have had a reoccurring problem with the brakes.  The brake pedal goes to the floor with little or no resistance, with resulting loss of brakes.  If I pump the brake pedal, there is little or no change, which rather eliminates the idea of an air-lock in the pipes or slave cylinders.  Apart from which, the brakes performed perfectly for the MOT.

 

I tried bleeding the brakes and all seemed fine for a day or two and now the problem has occurred again.  If I take the top off the brake master cylinder and pump the brake pedal, air appears in the master cylinder reservoir.  The fluid level does not appear to go up or down when the pedal is depressed or released.

 

Before I start another futile master cylinder strip down (I only recently replaced the master cylinder seals, though this is the original master cylinder), does anyone have any ideas borne out of experience?  The air bubbling up in the reservoir would presumably indicate that air is being drawn in past the piston and seal, into the cylinder - which presumably wouldn't happen if the piston has jammed in the cylinder.  I have serviced master cylinders before so am confident that I reassembled it correctly, but if one of the internal components has failed, could it lead to this problem?

 

Any advice appreciated..

Thanks,

Graham

[Pete Lewis]

from what you have found this does sound like the seals are failed ,

 

when you do a rebuild do make sure the pushrod has a small amount of slack when the pedal is back against the switch stop

 

these are a simple single seal cyl.   if its  main seal is  failing, the air trapped between piston and pushrod boot  get expelled into the reservoir on pedal returning as it bypasses the failed seal lip.

 

some boots have a small bleed hole to  solve  this 

 

 

Pete