I'm at my wits end here. Let's start at the beginning.... I've got a 48 1-ton dually. Relined brake shoes all the way around. All new wheel cylinders. All new 3/16" brake lines (I think that's the size... 3/8" fittings on them?) A new master cylinder (NAPA part #NMC M1880) and the ClassicParts 71-984 conversion bracket. With speed bleeders on each corner I have pushed through at least 5 pints of fluid. I am 99% sure that there is no air in the system. I have adjusted the bakes so that they are "too tight" just to rule out the cylinders needing more travel than I can provide. I still have almost no pedal and the pedal goes all the way to the floor. The push-rod on the linkage seems to push the master cylinder through it's entire range of motion. At the very bottom of its stroke I can see the bracket flex like it has bottomed out. The truck does stop but it doesn't "feel" right (or safe.) My mechanic friend is also scratching his head but is pointing his finger at either the wrong master cylinder for this application or incorrectly sized brake lines. We've checked (and rechecked and rechecked) and there are no leaks or places where air is getting into the system.
Because you are using a "conversion bracket" I assume that the NAPA part #NMC M1880 is not designed for a 48 1 ton dually. What is it designed to fit? And what size wheel cylinders are used on the vehicle it is designed to fit? What size wheel cylinders are on the truck? Bottoming out a master cylinder (especially under load) doesn't sound like a good idea to me.
The M1880 is designed for a 1970 C10. That has a front wheel cylinder bore of 1.125" and a rear cylinder bore of 1" I'm not 100% sure but I believe both front and rear on the 1948 1-ton are 1 3/8" The M1880 is a dual piston w/ a 1" bore and the stock MC has a 1.25" bore. It's possible that the 1" master cylinder is not pushing enough fluid, I suppose. Do you have any idea what would be another drum/drum MC with a 1.25" bore? Summit is only showing disc/drums in that size.
Late 50's GMC trucks (I don't have specs for your '48) used 1.125" bore master cylinders on 1/2 through 1 ton models with 1.25" bore MCs on 1.5 ton through next larger set of trucks. I think your mechanic is onto something with your current choice of master cylinder not moving enough fluid because it is under sized. The difference between the 1" that you are using and 1.25" would be 37.5%.
Thanks, Bill. I can find drum/drum master cylinders in 1.125 but I haven't been able to locate a part # for a 1.25" drum/drum setup. Any chance you've got some information to help me track one down?
Just spoke with Jeff @ CPP -- they make the adapter kit for the MC -- and he said that he doesn't think the bore has anything to do with my problem. He said they never use or recommend anything bigger than 1" for a manual setup. They only go up to 1 1/8 w/ a booster or hydrovac.
Jeff is the same guy that told me CPP's Toyota-based power steering kit would fit my '57 GMC V8. It didn't. Cost me $150 bucks restocking charge to return the kit.
The line size is not critical since the exact same amount of fluid moves through either which is how much is displaced by the mc piston in one stroke. Will they pump up with several strokes? Bill is absolutely right (and Jeff needs a new line of work) because you will NEVER get full pedal on a 1" mc in a system that was designed for 1.25". The pedal effort will be quite a bit more with the bigger bore but you will have pedal. To know for sure get a plug, or plugs if a dual cylinder, and screw into the mc where the brake lines go. They have plugs with a tapered bottom just for this at your local parts store. After plugs are in have someone hold pressure on the pedal and crack each plug to bleed. If you then have full rock hard pedal with the plugs in place I'd bet the mc bore is too small. We have a bleeder ball but with frame mounted mc's we usually reverse bleed the brakes which it is messy but effective.
Personal experience with CPP was miserable. They sent a brake setup clearly labeled for a '35 ford, and insisted it was for my truck. I had to threaten legal action to get them to take it back at full refund. With respect to the MC problem, is it possible that there is a problem with the residual valve in the MC? If you pump the brakes several times quickly, and can then build up pressure that may be the problem. Almost forgot, you can use a disc/drum MC with an external residual valve for drum brakes on the disc side. This should put out more fluid per stroke as well, (I think).
Thanks guys. With a plug in one side of the MC I can get good, HARD pedal. I can't find a 1.25" MC but I did order a 1.125". I figured it's worth the $40 to see if it makes it better. Certainly can't be worse. And no, I can't pump up the brakes. Even with 20 strokes of the pedal it will still go to the floor.
