okay this is my first post and i hope this is the place to go!

i just picked up a 88 B250 van and the brakes are shot... i replaced the rear wheel cylinders because they were WAY rusted and the master cylinder because i wasnt getting fluid to the rear brakesand most fittings and hoses...

puzzler is if i push the brakes fast i have great pressure on the pedal and stopping like it should be and it will hold in the same spot as long as i hold the pedal, when i push slow and soft the pedal goes all the way to the floor..

i pulled the bleeders out of the rear and am getting no fluid out of the bleeders.. i blew through the lines and they are clear and tried a vacuum pump and am still notgetting fluid out of the rear lines.. would the proportion valve be the culprit or the booster?

 

Your new MC is no good. This is common for rebuilt units if the bore was honed out, which is why I only use new ones.

 

thanks for the help i will go and get a new one and see if that fixes it.

also the other day when i was pushing on the brakes i could hear air escaping from the seal around the rod out of the brake booster, is that cause for concern too? thanks again!

 

The booster is supposed to be an airtight vacuum chamber. So yes, if it's leaking it will need to be replaced.

Follow these steps to test the booster before spending the money and replacing it.
1. Operate the engine at idle without touching the brake pedal for at least one minute.
2. Turn off the engine and wait one minute.
3. Test for the presence of vacuum by depressing the brake pedal and releasing it several times. If vacuum is present in the system, light application will produce less and less pedal travel. If there was no or little vacuum assistto start with thenair is leaking into the system.