Problems with hydro boost
 

Hello all, I seem to have a problem no one else has posted, or at least I can't find any info. I have a 1990 Chevy suburban with the diesel, so it has hydro boost braking system. Anyway I haven't owned it for very long but shortly after I started driving it every once in awhile the brake pedal would go to the floor. I mean straight to to floor and brakes would not engage. Let the pedal up and I had brakes again. Very random. I figured maybe a bad master cylinder, went to Napa and picked one up, bled new cylinder per instructions and then bled entire brake system starting at farthest wheel from master cylinder and working my way around the truck bleeding each wheel until I got clean brake fluid coming out and no bubbles.

Well worked for about a week then brakes went to floor just like before. Happened a few more times over the next couple weeks, usually when I'm moving slow thank goodness.

So then I changed out the hydro boost thinking something was bad in it. Worked for a few days and brakes went to floor again.

I bypassed the equalizing valve under the master cylinder thinking maybe that was problem. Nope still went to floor.

Took it to brakes plus, they said they bled the brakes again and everything was good. Nope after a couple days went to floor again. They didn't know what was wrong, and it never happened to them. Like I said it's very random. Sometimes it's good for a week sometimes it will happen several times on the same day.

Well I'm at a loss. Hoping someone here can help me out. Any