So is it possible to test a brake booster at the junkard with no source of engine vacuum?
A little history:
75 FJ40 w/ f &r disc conversion, Supra booster, original Master cylinder with residual valves removed.
My brakes began a slow fade the other morning while heading to work. I had initial pressure but then the pedal slowly bottomed out. I can hear a hissing noise from the booster when I apply the brakes. I took off work early and ran to Pick n Pull before they closed and picked up another dual diaphram booster from an '84 Supra. Swapped out the old booster and put in the new one. Sure enough, it still does the slow fade and hiss when I pump the brakes. :eek: :mad:
I start to second guess my diagnosis, so I bled all the lines just to make sure there wasn't any air trapped in the system. I also checked the engine vacuum and hoses for leaks. All is good. Slow fade is still there. Although there is no fluid loss or signs of leakage at the master cylinder the following night I decided to swap the original 75 master with a rebuilt Supra master I had laying in the shed just to make sure that fluid is not seaping from one chamber to the other. No change. :mad:
Figuring I just had bad luck and got a bad booster, I decide to swap in the single diaphram booster which I removed two years ago when I did the disc swap. It worked fine when I removed it. Guess what?! Yep. It is hissing and the brakes are still doing the slow fade. :mad3: WTF?
Good thing PnP is having their half price sale this weekend!

So, before I go back to find another booster;
1. How do I test it at the junkyard?
2. Is there something else I am overlooking?
3. Is it common for booster diaphragms and seals to go bad as they sit unused?

Could it be the line going from the manifold to the booster?

3 boosters all with the same symptons is highly unlikely. Possible, but I would keep looking for the problem elsewhere. To test a booster you need a vacuum source, like one of those inexpensive hand pump vacuum bleeders that have a gauge. You can get one and test all the boosters you have and then sell the good ones.

Maybe a piece of rubber hose and a funnel to make a stethoscope and see exactly where the hissing is coming from. Describe the fade. Is the assist there from the booster or does it get progresively harder to push the pedal?

Eskimo - I checked the line from the manifold with a vacuum guage. It's fine. I'm not losing vacuum from the engine.

SpaceGhost - I can hear air escaping from the piston connecting the pedal to the booster under the dash. In all three boosters. :( Maybe this is normal? I've never noticed it before, but I've never really listened closely. When I hit the brakes I initially get good pressure, then as I keep pressure on the pedal, it slowly drifts downward and braking effect fades also. It doesn't get progressively harder to push. I can hear and feel air escaping from around the booster piston as the pedal fades.

The way the pedal fades makes me think that there is air in the lines, but I've bled the system a couple times to no avail and the air leak in three different brake boosters has got me stumped.

I would still not assume the boosters are all bad, just so unlikely. If the pedal slowly makes its way towards the floorboards I would be looking for something hydraulic to be the problem.

These are the obvious things I can think of, so don't be insulted if you've already checked them.

Master cylinder although you have swapped a different one. You could rule the master and booster out completely by running a line from one port to the other, capping the system and retesting. Plugs would work too, but good luck finding some.

If the problem is downstream of the master, then loose fittings, punctured lines, or leaking wheel cylinders are a possibility. You should find some trace of fluid in this case.

I don't recall seeing anything that indicates you have rear disks, so I would be pulling the brake drums and checking the wheel cylinder's condition.

BTW, I don't think the hissing is right, but without a vacuum source and a guage it will be tough to eliminate the booster for certain. I know that some sound is expected when you depress and release the pedal. Not a hissing but more of the actaution of the booster (like a bellows).

Goos luck,

Not insulted at all thanks for the suggestions SpaceGhost. I'll try running a line from one port to the other.

Yes it's got the chevy rotors in the rear. Mini truck up front.

I haven't found any leaks in the system and my fluid levels are unchanged. This is something I check each time I open the hood.
I'll keep poking around and see what I can come up with. I'm beginning to think it may be a combination of a couple things.

I thought I'd bring this back to the top to let everyone know what the final diagnoses turned out to be.

It wasn't three bad brake boosters. It was two bad master cylinders. I took Spaceghosts advice, broke out the double flare tool and ran a short line from the front fitting to the rear fitting of the master to rule out the rest of the hydraulic system. Sure enough, the piston bottomed out after continued pressure. It wasn't real noticable at first when the master cylinder was seperate from the booster and it was just pushed by hand. With the booster attached it botttomed out fairly easily. Both master cylinders did this, even the Supra booster I rebuilt a couple years ago.

I've got a new master on order, so in the mean time I'm getting plenty of exercise commuting to work on the bike.
Thanks guys. :D

Thanks for the update, this is how we all learn! Glad any tidbit I offer is helpful. Where did you order the replacement from? We have a place here that is "Imports Only" and sells Aisin masters at substantially reduced prices.

Many have stated they get good results from the fj80 masters, I should check the prices and see how stiff they are. I recently picked up a gear reduction starter from them for $90 something. It was a rebuilt Bosch unit that appears to be brand new. The tag states rebuilt, but it is decieving to look at. Last one died after multiple dunkings in a silt/sand bottomed creek we play in. The core charge is $60 if you don't have an identical unit to swap. This is still less than other sources I have found.