More details on an master cylinder problems. These photos supplied by Chrsitopher Wilson in North Carolina, USA. The first photo shows disassembly with all internal parts in view. Click for larger images.

Picture below left shows a difference in seal cup size. The larger diameter seal (notice bulge) will initially drag on the bore causing the piston to stick in the depressed position. If you return the piston with air pressure and cycle it about 100 times it will eventually loosen up and return by spring force only (but it's a pain to have to do this with new parts).

Picture above right shows the TRW/Lucas flow control valve. This one (not as original) has inlet ports on the end with a check valve inside. The current TRW/Lucas units seem to have stronger springs to return the pistons, and this causes higher than normal residual pressure that can make the brakes drag. If it retains too much residual pressure you can drill a small slow return orifice low on the side (see prior article). Original style valve body has a slight ridge around the top end, small holes on the side, and a small rubber cup inside. Standard rebuild kits include that small rubber cup, but is not used with the TRW/Lucas control valve.

Pictures above show the primary pressure seal cups and the control valve washers. I have recently encountered premature failure of the Moss and V.B. primary cups (4 years and 1 year respectively) causing internal leakage back to the reservoir when the pedal is depressed. For the brakes this causes the pedal to gradually sink to the floor requiring another pump to restore pedal height to avoid brake release. For the clutch it causes the clutch to gradually re-engage while the pedal is held down. You can't double pump the clutch while the transmission is in gear, so you have to be quick to yank it out of gear to avoid killing the engine, and then don't hold the pedal down too long. I believe the Moss and V.B. rebuild kits come from the same supplier, as the seal parts look identical and are marked Made in England".

I have a vintage Lockheed master cylinder that has been repacked a couple of times in the past 10 years, after the first dozen years of trouble free service. I have never had any problem with the flow control valve in the brake side. This seems to be a fairly recent phenomenon with replacement master cylinders. The TRW/Lucas unit apparently has stronger springs than original which make it retain 12.0 to 12.5 psi residual pressure rather than the originally specified 8 psi.