Some might recal that I wrote:
>>My '57 MGA has the stock drum brakes on the rear but the DPO installed
early
MGB disc brakes on the front. The brakes work well but brake pedal travel is
excessive (pedal travels over half-way before it's solid). Pedal travel is
normal/short after one or more pumps of the pedal. The DPO says he just got
used to always giving the pedal a pump to bring it up.
Since there was over 10 years of very little use and one rear wheel cylinder
was leaking a little, I overhauled the rear wheel cylinders, brake master
cylinder, and clutch master cylinder. All appeared to be in good shape
except for deteriorating rubber. After getting everything back together and
bleeding the system the pedal travel is now just as excessive as before. I
have the rear drums adjusted correctly (one click shy of lock-up), the front
pads look good, I've bled the system over again, and there are no leaks or
loss of brake fluid, but pedal travel is still over half-way to the floor
before it's solid. After one or two pumps the pedal is solid with short
travel.
Is it possible that the stroke volume of the brake master cylinder is too
small for the volume of the cylinders on the MGB front discs? Is it possible
to bore out the cylinder and get oversized pistons, cups, etc.? I think
later MGA's with stock front disc brakes used the same master cylinder as
early all-drum models. Could there be that much volume difference with the B
brakes? Also, there is a non-return valve in the brake master cylinder which
is supposed to maintain some pressure in the system. If the system maintains
pressure, then volume of the master cylinder shouldn't really
matter...right??? Am I chasing the wrong zebra here?<<
Much thanks to all who replied with advice on this as it has me stumped.
My plan for now is to go ahead and rebuild the front calipers (seals) and,
while I'm at it and while Vic Brit has things on sale, replace the rotors.
I'll also go over all the lines again to see if I've missed a leak
somewhere. The master cylinder looked so good inside that I'm pretty sure
it's working properly. I'm still considering the possibility that the master
cylinder volume is too low for the B front discs. This still bothers me
though, because I keep thinking that if the system is closed and partially
pressurized (by the non-return valve) it really shouldn't have to move much
fluid at all. Right?
Does anyone out there have the actual spec's on cylinder diameters/volume for
A' and B's? The master cylinder is '57 A and the front discs are probably
'63 B.
BTW, some may remember my rough idle/high idle/SU tuning/colour tuning post
from December. I appreciate all the suggestions, but I haven't made much
progress (mostly because the garage is too damn cold and the wheels are off
for painting). I did check compression (160 on all 4), looked high and low
for vacume leaks (found none except for very slight leak at throttle
spindle), replaced ingnition wires (solid copper core), replaced coil (Lucas
sport coil), made sure vacume and mech advance works on distributer, reset
the timing, and retuned the SU's. And then............no change :( I'm not
sure what I'll try next, but I'm going to wait for better weather.
Keith Baer
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