Wondering if anyone has seen this. (And yes, the truck is an 85 but the brake systems are virtually the same so I'm posting here )

Anyway, for some reason last week I noticed that my brakes pretty much sucked. I was having to push harder than I remembered, but I wasn't really sure when this started happening. Figured brake booster was the first thing to check. The pedal wouldn't depress when the engine started, and the braking felt the same with the engine on or off. So it is safe to say the booster was not doing its job.

First thing I wanted to check was the check valve and make sure vacuum was getting to the booster. Engine off, I crack the hose open at the booster, and there's a big intake of air. So the booster was holding vacuum, and did so for half a day. And the check valve was working as it should.

I next test the vacuum is getting to the booster by starting the engine with that hose disconnected. Big vacuum leak, idle way low, plenty of suction at that hose. So that is working as it should.

Next, I was planning on putting a dual diaphragm booster on there anyway, so I put it back together and head to the local junkyard to hunt around.

Lo and behold, when I crank it up, the pedal depresses, and the brakes are working like magic. The truck seems to have fixed itself. I can now lock up the 35s on pavement. No need for any brake upgrades! (Well, I will do the Fj60 rotor/ifs caliper/big bore MC swap eventually, but for now the need is not immediate.)

Soo... thoughts on how this happened? Clearly popping that hose off and releasing the vacuum did something to get that booster working again. But, from the internal diagrams of brake boosters that I can find online this doesn't really make sense.

 

My 86 runner if I was driving at higher rpms would just start applying the brakes without me touching the pedal. Must be a diaphragm in there that gets out of place somehow. I wonder if yours just got reset to the correct position when you disconnected the hose.

 

Sounds like a broken return spring in your case?

It weird with mine because the pedal has always returned to the proper spot. So maybe the diaphragm separated from the pushrod inside the booster somehow? Something goofy is going on in there, i'll be paying close attention...

 

Mine turned out to be a bad booster but I would use caution for sometime until you find out what it was or it may have fixed itself.

 

Stuck check valve on the booster vac line.

 

Thought about that. But, I cracked the hose right at the booster, and air came in. So the booster had vacuum and the check valve must have been working. When I blew into the hose air passed to the intake, so that should show the check valve is good, yes?

Maybe it was partially stuck, so the amount of vacuum was reduced, and then when I blew into it I freed it up?