The Stovebolt Site Forums The Parking Lot Tons 'o Fun Brake woes!!

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.

Anyone have any thoughts on the master cylinder or the brake line sizes? What else could possibly be wrong?

On my 1 Ton the brakes are the original type. There is no way that I could make any of the mounting hardware flex. Do you have a different master on yours?

In general, if everything else is good, if the shoes aren't adjusted correctly, you should get a pedal pumping it twice. If you can't do that then it is something else.

Sounds like it to me as well.
What is the original MC bore diameter and the new one? likely you have an undersuize MC and it takes a full travel to move enough fluid to activate he brakes. The correct bore size will move enough fluid to work the brakes and have plenty of travel left to spare.

Also you said the bracket is flexing, that doesn't sound good, look at fixing/bracing that.

Grigg

Hy 48bomb, your stock master cylinder had a bore of 1 1/4", you have replaced it with a master cylinder with a 1" bore. I believe Grigg is on the right track, your master cylinder bore is probably too small to move the volume of fluid required. I would look for a master cylinder with a 1 1/4" bore, hope that helps.

Sounds like it's worth a try at least. I used the master cylinder from a 1967-1970 C10.

Anyone know another dual-piston, drum/drum master cylinder?

Hy 48bomb, you could use a master cylinder intended for either full disc or disc/drum and install resisdual pressure valves in the lines/ports that were intended to feed disc brake calipers. I did find a 1 1/4' bore master with the same porting as the one you have currently, Raybestos p/n MC36359, currently available at RockAuto. I have no idea of the original application, so you would have to find that out and plan on the resisdual pressure valves if it is a disc or disc/drum original application, hope that helps.