What is the master cylinder bore diameter of a stock '48 Ford passenger car? I have '48 brakes on the front and 8" Mustang drums on the rear. The pedal has 7 to 1 ratio but requires a lot of effort, and the rear only locks up. I plan to replace the master and add a p'valve.
Master under the floor? Don't forget the risdul valves. The set I had on my 32 was the same as your using. Worked great for me. Didn't have a porportioning valve. Might drive it first.
The '48 front Lockheeds probably need a re-adjustment or rebuild, but your rear axle Bendix drums will still be better brakes. A prop valve may be needed in this case, but a conversion to Bendix brakes up front would be a great up-grade that would better balance the brake system.
It would be so much easier to share knowledge and give advice if those asking the questions would include some details about the car, but this one isn't too hard to figure out...I don't think. Model A (in avitar photo) with underfloor pedal assembly currently fitted with a single-circuit '48 Ford master cylinder, '48 Ford drum brakes on the front, Mustang drum brakes on the rear. Probably no residual pressure valves or proportioning valve, right? This was a pretty common setup in years past as was the poor brake performance the OP mentions. If this was my car I'd follow V8 Bob's advice and rebuild and properly adjust the brakes on all four corners. I would replace the single-circuit '48 Ford master cylinder with a dual-circuit Mustang-style 1" bore master and split the front brake circuit away from the rear wheel brake circuit in the process. I would add two 10 lb residual pressure valves, one in the front circuit and one in the rear circuit. I would also add a manually adjusted proportioning valve in the rear circuit between the residual pressure valve and the rear brake cylinders. As V8 Bob pointed out the modern rear drum brakes are going to require considerably less line pressure to activate than the '48 Ford front brakes due to differences in design. So, with the above parts in place I'd begin by closing the proportioning valve all the way and make a couple of stops using the front brakes only. Then I'd begin to open the proportioning valve a little bit at a time until the rear brakes can be felt contributing to the stopping effort BUT NOT LOCKING UP. If the rear brakes lock up, I'd close the proportioning valve a little at a time until they no longer do so. This is largely a matter of trial and error. If the pedal pressure is still too high I'd go with the same Mustang-style master only with a smaller 7/8" bore. It is unlikely that fluid volume will be a factor unless the brakes themselves are not properly adjusted. Hopefully this brief tutorial will help.
There should be a residual pressure valve in the ‘48 master cylinder unless it is defective or has been left out during a prior rebuild. Of course, if the m/c is replaced with a disc/disc m/c, RPVs will need to be added to the system. Even a later model drum/drum m/c might need them added. But I agree with V8Bob, upgrade the front brakes to Bendix brakes, either Lincoln or F-1 pickup and get a lot better results for your time, effort and money. I also would not install a proportioning valve in that case until proven necessary. If after adding Bendix to the front, the rears still are too ‘powerful’, change the rear wheel cylinders to a smaller diameter piston.
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