I replaced a 7/8 bore single circuit master cylinder on a O/T car(1952 MG)with a one inch bore single circuit master(new, speced for a 1965 Jeep cj5)and now I have an extremely hard pedal and it doesn't want to lock the wheels. The shoes are moving against the drums but the wheels won't lock tight. Does the difference in bore size cause this? I know the MG is O/T but swapping master cylinders is something HAMBers do a lot so maybe somebody has the answer.
yes, the bigger bore is moving more fluid with the same amount of pedal travel as the smaller bore, so it takes more pressure to push the pedal.
7/8 to 1" is a huge change for older type drum brakes. Example: VW upgraded the beetle brakes in 66 by a 2mm change in the MC bore to reduce pedal pressure. I still would want to know if each piston in each wheel cylinder is not frozen or sticking, though. However you say the pedal is rock hard "now", so that backs up the theory that the bore is way too big.
As near as I can tell the wheel cylinders are ok. The term "huge change" is alarming. Is there any kind of fix for this? A repro master cylinder is over $300. The jeep master which bolted up just fine was $51.00 brand new.
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