Technical 52 chevy 4 wheel disc brake problem

got a question for all of you. I'm finishing up the power disc brakes after bleeding I notice I have to pump it a few times to build up pressure and when I do get pressure I depress the pedal few seconds later pedal drops down to the floor agsin until I pump it up again 3 or 4 times. Anyone got an idea?

 

Is it under floor or bulkhead mounted master cylinder..? If its under the floor the fluid could be running back to the m/cyl, might need a set of valves to stop it

 

Master cylinder and booster are mounted on my firewall

 

Single or dual master cylinder, and from what vehicle?
Front disks are from what vehicle?
Rear disks are from what vehicle?
What other components are in the system? Proportioning valve?

If you can pump and build pressure to get a solid pedal and it happens repeatedly I'd suspect there is still some air in the system.

 

Did you bench bleed the MC?
How are you bleeding the system?
Are the caliper bleeders the highest point of the caliper?
Crush washers at banjo fittings?
Verify all connections are tight and leak free.
Just because an MC is new doesn't mean it's good.

If the engine is running and the booster is getting vacuum, try this.
Engine off. Bleed down the booster with several pumps. If the pedal is steady, keep it depressed.
Start engine. Pedal should drop ~1" If it floors I would suspect a bad MC internal seal.

 

Do you have GM metric era front calipers?

If so, you probably have the wrong ones. It is a common problem.

 

I hope the OP is OK.

Maybe he could not stop.

 

Well....his hanlde does say 'Illest'. Maybe he's sick? Hope not...a lot of bad stuff going around these days.

 

True that. I do hope that he's okay.

 

we have had our share of brake pressure problems that were always linked to the proportioning valve. we stopped using them and have good pressure without any lock up or pulling. we use tee fittings and the correct master cylinder for the type of brakes. just my two cents.