The Stovebolt Site Forums The Shop Area Driveline Dual master cyclinder issue

I installed a dual master cylinder with booster on my 1957 Chevy truck. The running gear has been been converted to NAPCO. I bought the master cylinder off eBay (good price-but apparently not a name brand - it came with no directions or any paperwork}.

The problem is that the brake requires a lot of effort to stop the truck. Also, it seems like there is little pedal for the first half of travel, then it appears to begin actual braking. Its almost like a "two step" travel. If I hit the pedal hard it does lock the brakes. It is very scary to operate like that. It is drums all around, with no proportioning valve. The brakes have been bleed multiple times and seem free of air. The boost appears correctly installed (connected to Edelbock carb-appropriate take-off} with no leak.

I'm wondering if it is just a poor master cylinder, or did I install it wrong. Has anyone ever encountered this or have any suggestions?
Thanks,
don

Hy Donsz, if the master cylinder was intended for a disc brake application, then you will need to install a residual pressure valve in each brake circuit, I believe a 15 p.s.i. valve is the one specified for drum brakes. The other thing that you should make sure of, is that your brakes are adjusted properly, hope that helps.

How confident are you on how well you bench bled the unit before installation? I have experienced ones that took a lot of bleeding to get all the air out of the system.

Sweet:
I didn't bleed the master cylinder beforehand. Can I still do it while it is installed?
don

Do you have free play on the rod going into the master cylinder?

I don't have much free play on the rod. However, when returning to the non-braking position, it doesn't always go back to the absolute top position. Not sure if that is an mechanical or actual brake cylinder issue. I've never bled a master cylinder, how is it done.
don

Do you have a return spring on the brake pedal? If not install one. Then make sure you have 1/4" free play for the rod before it makes contact inside the master cylinder.
If you don't have a spongy brake pedal you don't need to bleed master cylinder. If you need to bleed master get a couple short pieces of line and screw them into the outputs for the master cylinder and bend them so the other end goes up and into the reservoirs of the the master cylinder making sure they are below the fluid level. Once they are in slowly pump the master cylinder the full travel, usually take 5-6 strokes to get it done.

Thanks for the inputs. I did bleed the master cylinder as per Don's procedure, but it made no difference. Also, it appears (3B) that a drum-to-drum unit doesn't require a proportioning valve. I've come to the point where I don't have confidence in the master cylinder itself. As I stated before, I bought it off eBay and it was much cheaper than the normal price. I didn't get any directions with it and it wasn't even packed in an "original" container, although it did appear new. The irony is that I have never scrimped on restoring this truck, and this was the one worst place to do that. I have ordered the correct complete unit from CCP and am hoping that will solve the problem. At least, I can have more confidence in the part itself. I have heard good things from those that have replaced their original master cylinder with the dual/boost unit. The way mine operates is an accident waiting to happen. I'll let you know how it turns out, again thanks to everyone for the help.
don

Hy Donsz, I realise that a drum/drum system does not neccesarily require a "proportioning valve", but if your master cylinder was designed for a disc/disc system, there will be no "resisdual pressure" valves built into it. Resisdual pressure valves are required for drum/drum systems to function properly, without them the first pedal pump or portion will only bring the shoes into contact with the drums, the second pump, or portion of the first pump will start applying the brakes, hope that helps.

I have a power dual master on my NAPCO. You need to have a drum/drum setup. The ports are specific to the front and rear brakes, maybe they are switched? The port closest to the booster goes to the front brakes. I don't have a proportioning valve. On hard braking, the rears will lock but that is to be expected due to weight.

Oh, and get a big strong return spring.

3B & jomichael: I've lost confidence in my current master cylinder, I'm beginning to doubt that it is a drum-to-drum.
3B: interesting comment on the pedal/braking sequence. That is what appears to to be happening to me. I did order a complete new drum-to-drum assembly from CCP, so I hope that solves my problem.
jomichael: thanks for the comment on the return spring, I didn't think I needed one, but apparently I do.

Get a master for a 70 C10 with drum/drum brakes from any local parts house for probably simular money that you paid in shipping for the E-bay one,
Scott

We really need a place to put these little tidbits of info. so we don't spend more money than necessary! This was about 2 years too late for me. Ugh.