Hydroboost brakes acting up

[slow poke]

I am looking for someone to confirm my suspicion, and help me buy the best replacement parts. I have a 71 bronco with a stock disc/drum master cylinder and an astro-hydro boost set up. The brake pedal has been creeping towards the floor when holding at a stop light. The dash brake light went on, but went off after a stab on the brake pedal. Today the brake pedal went all the way to the floor during normal driving. This all started when I was pulling someone out and had my brakes pushed down hard while winching.

I suspect the master cylinder seals are shot, as there is no brake fluid leaking any where. Is there any chance it could be the older used hydroboost unit?

I am planning to replace the master cylinder, but wanted to get some feedback before I start replacing one part after another. Assuming it is the master cylinder, are there any good candidates for new ones? My old master cylinder and the hydroboost has different bolt spacing which has been re-drilled so I can go either way on the master cylinder. I may want to keep the door open for a rear disc upgrade someday also, but I would hope a disc/disc master would work for a disc/drum.

I miss my brakes…wow that sounds stupid, but the hydroboost was a huge improvement to my brakes. So much so I may not do the rear disc swap if I can get them going well again.

Thanks for you thoughts and advice.

Dave

[Daddy4zack]

Re: Hydroboost brakes acting up

try a new M/C.. 40.00 from a parts house.. 1 1/8th bore!!

[Bronco Brian]

Re: Hydroboost brakes acting up

Mine preety much did the same as yours but it was the H block / Proportioning Valve I am going to start by taking it out of the loop.

[904Bronco]

Re: Hydroboost brakes acting up

I am running a Astro van MC with the Astro van Hydroboost unit (1995). The Van MC is 1 1/8" vs. the stock 1". Firmer pedal, still stops the same. Plus you do not have to modify the master to install.

Sure sounds like a bad master, bleeds down while holding pressure and bumps up if you pump it...

[slow poke]

Re: Hydroboost brakes acting up

Thanks for the help and thoughts. Is there a way to tell if it the master or the proportioning valve? A bit more info based on 904's note is that the pedal used to get better with a pump, but would slowely go down at a stop light, and now there are really brakes for just a few seconds and then they go to the floor. The brake warning light comes on frequently, but will go off with a jolt to the pedal.

It seems like if it was a bad proportioning valve that either the fronts or rears would still work, and the pedal would have pressure. Does this make sense?

Thanks, Dave

[Fordfreak]

Re: Hydroboost brakes acting up

plug and bleed the lines out of the master. if it still does it it has to be the master. the hyd-boost leaks out the back and causes an oil leak down the fire wall if it's bad.