piss poor brakes

[whiskeymilitia]

1955 Chevy Truck. Heres what I have done so far. Installed a power booster and master cylinder , front disc brakes and 10 bolt chevy rear with drum set up . Installed residual valves and proportioning valve. Bench bleed the master cylinder and installed and have bleed the brakes a half dozen times. I still can't seem to get a good pedal. If I pump the brakes once the truck stops ok . But other than that the pedal pretty much goes to the floor with little stopping power. Any ideas ? Thanks

[CPPAaron]

Whenever you have to pump that is a sign of air. If you run an isolation test (start pinching brake lines) and the pedal comes up it has to do with a wheel component. If it still does not change the pedal feel it may be a master internally bypassing or something.

[whiskeymilitia]

Aaron thanks for the reply. Another question, how much rod travel should there be into the booster. Im using the stock 1955 Chevy truck pedals and was wondering if maybe I wasn't getting full piston travel into the master.
Thanks DAVE

[CPPAaron]

Between the booster and master you want to have a 1/32 to 1/16 of an inch of freeplay. Nothing more.

[ProMetalShop]

Hi Guys
I have come across that often even with all new components. I dose get frustrating, even after having help come to my shop and help with vigorous bleeding. I always start a the right rear and work to the left rear and then to the front right (passenger side) and finally the left front (drivers) . With all new components its even easier to get a small air leak somewhere. A friend brought a bleeder over that was air powered and boy did that make a difference. Mine also had residual valves do to the fact I installed a under the floor pedal assembly, and did not want the brake fluid draining back in to the master being lower than the calipers and rear wheel cylinders. I have a customers car that each summer we would have to bleed, after sitting all winter and all those components were brand new. I bought a new master two years ago and have not had to bleed it again. My point is it can be something we fail to do or see over the whole but every once in a while it can be a faulty components.

[JR.]

Prometal is dead on correct with the bleed procedure. One other thing I heard works is to jack the vehicle from the frame and let the wheels hang as low as possible to get them below the master cylinder. From what you describe it does sound like a faulty master cylinder though.....It does happen...:mad:

[wildearp]

Dot 5 is hard to get the air out of. Is that what you are using?

Are you sure the stroke of your brake rod in the master cylinder is correct? If it is preloading the master cylinder, you will not get a good bleed.

Under floor mounted master cylinders are particularly difficult. The problem is getting enough fluid flowing fast enough to push the air bubbles out before they back flow into your lines toward the master cylinder. Either a vacuum bleed with a large reservoir and a good means of keeping the master cylinder full of fluid is necessary, or you need a proper pressure bleeder. A good one can be quite expensive. This may be where you want to go to a brake shop that has the expensive unit and just pay labor, or search out a friend with a good pressure bleeder.

I have gone through this several times. In one case I had a leak that was almost undetectable. Look at your fittings very closely. Even the slightest leak will put a bubble in your line.

[Jetseatman]

I had some similar problems with my 56 Ford that I put a power brake setup on. The rod between the master & the booster has to be adjusted to just the right length. In your case, I would adjust it out 1/16" - 1/8" then try it. If that rod is too long, it will build up & drag the brakes. Too short & you'll have to pump 'em. Same thing goes for the rod to the booster from the pedal.