brake problem

[tfinley]

I have a 1979 dodge truck. I replaced the brake booster and the master cylinder. When the truck is not running the pedal is hard and seems to have good brakes. When the truck is running the pedal goes to the floor almost. The brakes work but something is wrong. I did bench bleed the master cylinder and bleed all of the lines. Any thoughts?

 

[GEO]

Check the line from the engine to the booster for leaks.

Brake clean spray works well for this.

Or you could have a bad booster

 

[tfinley]

the booster is brand new along with the check valve. The hose from the booster to the intake is also brand new.

 

[74dusterman]

Is the master cylinder the correct one for the drive train you have? I had the same problem with a truck I recently bought. The previous owner put a 86 D150 body on a 77 W200 drive train. The master cylinder for the D150 did not have enough volume or pressure to fully operate the brake system for the W200 drive train. I figured this out the hard way thru process of elimination and it cost me a lot of money. Needless to say I have all new brake components on that truck now and not because I wanted to.

 

[Jo Diesel]

You might need to adjust the rear brakes. This is common when replacing master as old probably did not actuate them properly. And you probably still have air in the system. When you have vacuum there is enough pressure to compress the air.

Try loosening a bleeder then put your finger over it then have some one pump the brakes. I start with closest to master and work my way back. This is opposite of what is taught but it works better.
Air finds its way to high spots in the brake lines and sometimes is a bieotch to get out.

A friend of mine is a Toyota Master Tech and he uses a small pump up yard sprayer and pressure bleeds up from wheel cyl. Empties master then as soon as bubbles stop he goes to next bleeder. Does complete bleed job alone in less than 30 mins with less than a quart of brake fluid.

 

[Dartswinger70]

Is there a rod that needs to be adjusted from the pedal into the booster? It may or may not be, I am just taking a guess. You may want to check the book on this. The above posters are also correct about the right booster/ Master cylinder etc. being correct. With brakes the diameters of the pistons in all the components have to match. You run into problems converting from drum to disc, and from manual to power brakes if you mix hydraulic components. if you are sure the parts are correct, look into the adjustment of the rod going from brake pedal through firewall into Master cylinder. Just a quick .02 while I was here. Good luck!

 

[AJ/FormS]

JoDiesel probably got it, that the rears need adjusting.Does the pedal come up, if you stroke it two or three times with the engine running? If yes, then the rear adjustment would be my first go-to.