Brake Booster ID/Problem

[slowcpa]

can anyone recommend a direct fit replacement for the below 5"x7" aftermarket brake booster? PO put discs on the front 20 years ago (ive had it for a week) and the brake pedal bottoms out when the engine is running. the pedal is rock hard when the engine is off. most of the posts ive read suggest a bad booster. ive checked for leaks and dont find any moisture or weeping seals (have not inspected the drums). ive replaced the proportioning valve (it was leaking) from a little hole in the side of the rear circuit. i currently have the plastic bleeding tool screwed into the prop valve. i clamped off the vacuum port and the brakes remain firm while running. i dont know how a faulty booster would cause this but im willing to be educated...im kinda getting frustrated and if more than one person told me shaking a dead chicken at it would help i would consider. pics...

 

[SLOWnEASY]

Looks like Broncograveyard, Wild Horses and Toms offroad has a replacement aftermarket for ~$100. Or get a whole setup for under $400 from Wild Horses.
https://shop.broncograveyard.com/1966-77-Bronco-Power-Booster/productinfo/12502A/
https://tomsoffroad.com/collections.../76-77-ford-bronco-power-brake-vacuum-booster
https://www.wildhorses4x4.com/76-77-ford-bronco-power-brake-vacuum-booster-oem

 

[4xfun]

The shape looks like one that aftermarket conversions use. Talk to Pirate Jack. Their website is https://piratejack.net/

 

[slowcpa]

thanks, im hoping to not have to rebend the lines or get a new plunger. any idea on a part # for the one i have? i would love to support any of those vendors but the ones you linked look to be 8" and this one measures out to 7". does that make a difference

 

[SteveL]

The original for that swap were like a 78 mustang 2 booster. You could try taking it to a local napa and having em match it up.

 

[DirtDonk]

Either the posts you have been reading are wrong, or you have been misreading them.
From what you describe, your booster acts perfectly fine.
They are rock hard when the engine is off and there is no vacuum. That’s good.
They assist when vacuum is applied and that sounds good.
Perform those same tests and hold your foot on the pedal with some pressure. Does it stay where it is, or does it slowly work its way down?

Sounds like you have either a mismatch between the booster and the master, a leaky master or other hydraulic component, or something else is going on.
Not limited too, but including your back brakes not adjusted properly.
As the shoes wear, and if the auto adjusters are not functioning, too much travel in the rear will end up being too much travel at the pedal.
Easy enough to check that, by applying the parking brake and seeing if the travel of the pedal changes.
Speaking of the parking brake, how many clicks does it take to actuate them?

It’s easy enough to pull the master cylinder away from the booster slightly to check for clearance.
The rod from the booster must almost touch the back of the piston when it is first attached, or you will have excess pedal travel.

And last, but not least (for now anyway) check that your front calipers are not touching the knuckles.
If it is the less common Ford conversion, this is not an issue. If it is the far more common GM conversion, it’s a definite issue to be confirmed.

 

[SLOWnEASY]

Rereading your original post do you have a problem with the brakes with the engine running; Or have you noticed something that you're concerned could be a potential problem? To go further with what Dirtdonk is asking, try this test.

With the engine off pump the brake pedal a five or six times until it is rock solid. Now step on the brakes and hold the pedal firm like sitting stopped on a steep downhill firm. While holding the pedal down start the engine and observe the pedal. What happens? Does the pedal drop an inch or two? Does the pedal immediately sink to the floor? What happens.
Something else I remember learning years ago is that a power brake system, well at least a vacuum assist power brake system can sink to the floor if you pump the brakes in a certain way. With the engine on step firmly on the pedal. Then lift your foot slightly but not all the way up and then step on the brakes again. If you do that correctly or rather it's incorrectly, the pedal will slowly get lower and lower until it's on the floor. Are you potentially doing something like that in trying to investigate whether or not your power brakes are working?