I just bought a 78 f150, and when running, the brake pedal gets sucked in. It releases a bit when the throttle is revved. Plus, the brakes are very weak. I've checked the vacuum hose that connects to the booster (pulled it off) and it has plenty of suction. Brake fluid in the MC is full. No leaks. Any advice? Truck is not driveable. Thanks!

 

I'm having the same issue with my truck after rebuilding. New MC, new lines, new calipers, new rear brakes. I've bled it three times and all is well, but as soon as it's running and I press the pedal, it stays put.... aggravating

 

That's a sign of a booster not releasing vacuum after it has been applied.

 

so booster is shot?

 

That's what mine was doing.... I could barely drive it around the block because the brakes were locking up. Right after a drive, I'd jack it up and it required significant effort to spin. Let it sit a few hours and the applied vacuum would bleed off and act "normal" where I could spin the front wheels with ease.

 

Yup, sounds like my problem. figures the one thing I didn't replace.

 

Yep.... Likewise.

Welcome to the club. I wasn't the first member and you certainly won't be the last.

 

Does this apply to me as well? I have to stand on the pedal to get the truck to slow down. So, just replace the booster (get the one with MC already attached), and bleed the brakes?

I assume I can just bleed from rear right, and move on up? Anything special about these models?

 

That's how mine was, I replaced the booster and the mc. The new mc was bad so I put another one on then went for a test drive and blew out the rear brake line. Having fun doing that now.

 

In your case, it sounds like your booster isn't doing anything. I bet it's got a blown diaphram. I'd just replace the booster if your MC is fine. No need to spend the bucks.

Be doubly sure the rod between the booster and the MC is property adjusted.

Bleeding is always RR, LR, RF, LF.

 

Sounds great. Is there a thread anywhere on 1978 booster replacement? I've seen it for 80s trucks, but it looks fairly different.

 

Also, I assume the rod adjustment is in Haynes? I just ordered it, and am waiting for it to arrive

 

how do you make sure the rod is adjusted right? I always wondered if it was

 

http://api.viglink.com/api/click?for...FMMBAK-1r1.pdf

 

I can see how having the rod too far out would hurt,, it could keep the rear seals from seating and always allowing air to bleed into the chamber. I'll check it and see if that solves the problem. thanks for the link ultra.