Power brake booster won't hold vac

[John Larkin]

I think I've been chasing my tail. When I powerbrake to build boost on the converter, I lose vacuum in the brake booster almost instantly. I can let off and brake booster quickly returns to normal. I have tested the check valve in the booster and it holds rock solid 25 in/hg on a Mity-Vac, no decay. What are the chances that my booster has a leaky diaphragm and can it be tested? When I pulled the check valve out after the car sat for a week, there was no burst of air pressure. I should retest it after running but this all just ran through my head while I'm here at work. Any thoughts?

I also have a very low pedal. It seems fine but just having so much travel in it before engaging the brakes is a little concerning. Maybe this is all related: low pedal, can't hold vacuum. Hmmmm....

 

[AsphaltAnihil8r]

Originally posted by John Larkin
I think I've been chasing my tail. When I powerbrake to build boost on the converter, I lose vacuum in the brake booster almost instantly. I can let off and brake booster quickly returns to normal. I have tested the check valve in the booster and it holds rock solid 25 in/hg on a Mity-Vac, no decay. What are the chances that my booster has a leaky diaphragm and can it be tested? When I pulled the check valve out after the car sat for a week, there was no burst of air pressure. I should retest it after running but this all just ran through my head while I'm here at work. Any thoughts?

I also have a very low pedal. It seems fine but just having so much travel in it before engaging the brakes is a little concerning. Maybe this is all related: low pedal, can't hold vacuum. Hmmmm....

IF you are pretty possitive that the check valve is holding ,then the only other reason is the diaphram,JMHO...
Dennis

 

[Evans Ward]

Run the Mity-Vac pressure check straight to the power booster grommet where the check valve would be. What I mean, is to take the check valve out from the power booster and run the MV fitting into grommet hole with a tight seal. Pump it up now and see if it holds a vacuum. If it doesn't, then the bladder is ruptured. I chased this same problem this week on my 70 GS car and determined that the power booster bladder was bad. I've heard of people repairing the bladder with a bicycle repair kit but I couldn't figure out how to open it! I think you have to have special tools to get into it. Easiest route is just to replace it and a used good part will suffice here. HTH.

 

[John Larkin]

Thanks guys. I just pulled vacuum on it and it decays rapidly, unable to pull over 5 in/hg on the Mity Vac. I'll be shopping for another booster (with Mity Vac in hand). I shoulda bought one of these things a long time ago! Kinda what I said after I got my ScanMaster.

 

[Roger]

After 3 lifetime warranty rebuilds developped bladder leaks plus the original junkyard unit leaking, I went back to the PM.
Lifetime warranty PM was $199 & I had more than that in the vacuum conversion.
A leaky bladder also changes its rate of leak with each depression of the brake pedal, making IAC setup impossible. When I finally pulled my vacuum unit off, the TR was ideling at 2k - or 70 mph.
I was working on a vacuum boost pump (more $), whe I gave up on the vacuum conversion.