Master cylinder, hydraulic abs?

[mussey ex]

Brake issues,

Picked up this 94 xlt for free a few months back, replaced all the brake line as it was rotted out. Brakes have been good for a month, last week the pedal started going to the floor periodically, now all the way. Not loosing fluid, seems like only the rear brakes are stopping me. bleed the system and no change. Sound like a master cylinder or perhaps the ABS hydraulic unit that I have no experience with?

 

[Anime]

If you push the pedal down, and it sinks to the floor, I'd say master cylinder.

The ABS unit would only affect the ABS operation, not general braking.

 

[Mettler]

Ive seen this before. It may be the master cylinder. It may be leaking into the booster tank. You have to take the master off and suck out the fluid inside the booster. You can check the booster without taking the master off. Take the vacume check valve out of the booster and stick a wire in to the tank like a dipstick. Ive used a turkey baster to suck brake fluid. Works nice. I just saw that you said that it doesnt loose fluid. Sounds like alot of air getting in there. Possibly a cracked rubber brake line. or maybe you didnt get a line completely tight.

 

[mussey ex]

No fluid loss at all. Checked all connections. With vehicle off, I can press the brake pedal twice and have good brake feel. As soon as it is started, the pedal goes to the floor.

 

[chetzar]

Sounds like its your brake booster, I had this problem awhile back and replaced my MC, no go, replaced the Brake Booster and all was well in the land of breaking. However, I dont remember what the test was, with the car on my dad did something and had me press the pedal to see if it stiffened up and it did, said the brake booster was bad, maybe he pulled the vaccum line off it? never really asked lol.

 

[Mettler]

brake booster check valve

 

[mussey ex]

How does one check it?

I've been doing some reading, and i'm wondering if the ABS hydraulic unit has air in it, Which I have come to find out, traditional bleeding methods will not remove the air from it, It must be brought to a garage.