1987 alfa spider brake booster works when it wants to

Intermittent problems are harder to diagnose than things that are simply dead. You might have a flakey vacuum connection somewhere: cracked hose, cracked connector on booster, bad one-way valve in manifold, etc. I'd start by inspecting all those components for cracks, loose connections, etc. After that, you might try wiggling the hose while the engine is running to try to find a place that produces a sucking sound.

I guess it's possible that something internal to the booster is intermittent. Don't know how you would diagnose that, other than rebuilding/replacing the booster and hoping that fixes it. So I'd try this last.

Does the engine run any differently when the booster isn't working? A major vacuum leak would affect the idle.

Could be a bad check valve and/or a leaking booster valve. Does the booster hold vacuum after you shut the car off? If no, check if the metal check valve at the intake manifold is working.

If it's not holding vacuum and the check valve is good, probably a leaky valve in the booster.

Thanks guys for the tips. I haven't had this issue before. Either the booster works or it does not.

try this,, plug the hose from the manifold to the booster,, so the hose only is connected to the intake manifold, plug that hose drive the car.. you might find out that stopping a light weight car with 4 disk brakes is not that hard.. it will be a adjustment in foot pressure, but not hard to get use too

Bought used car with motortrader had the front brakes sticking on at times. replaced rubber hoses, cylinders, shoes, Drums around. got a big deal chrome booster and cylinder. I'm replacing everything I got from them. I wish I could get american parts for the car. or south american parts would be better than China. lately found the master leaking into the booster. ordered a new set,chrome,china,lets see. FRUSTRATED WITH JUNK

are you in the right place?

Well I figured out my problem. I pulled out my brake master last night and saw brake fluid inside the booster so the master was leaking slowly from the back and messed up my booster rubber just in case others have the same issue down the road. The master was low when I bought the car and I added some and kept an eye on it and it didn't seem to drop much but I only put 100 miles on the car or so.