What Causes This?

[helpme]

Was parking on a hill( about 10 to 15 degrees). As soon as I switched off the ignition, the brake pedal went hard and the truck began to roll backward. I immediately disengaged the clutch pedal to stop the rolling.
Is this a bad master cylinder, bad brake booster, or vacuum problem elsewhere?
It is an old vehicle, a Vulcan 3.0 with 277K miles on it. The master cylinder was replaced about 2 1/2 years ago.

 

[Ranger850]

Bad Booster?

 

[sgtsandman]

Brake booster comes to mind for me as well. That an using the parking when parking and shutting off the truck. Depending engine compression to hold it is an accident waiting to happen.

 

[dvdswan]

One-way valve for the booster. Or the booster itself if the valve is internal.

 

[Dirtman]

Loosing vacuum somewhere from the check valve up. I believe on those the check valve is just the piece on the end of the vacuum line that plugs into the booster. But the booster itself could also be leaking somewhere internally.

 

[RonD]

+1 to all the above

Pedal gets hard because you are losing the Vacuum assist from the booster as soon and engine is shut off
That shouldn't happen, the booster is so large so it can hold 3 "assisted" pedal pushes in reserve, in case engine should stall while driving, gives driver assisted braking to pull over safely and stop

On the booster there is a check valve, just looks like a 90deg fitting, which it is, but there is a check valve inside it
It just pulls out, the check valve holds vacuum in booster so engine can't use it when its vacuum level changes

But booster could also have a leak in its internal diaphragm
Follow boosters hose over to the engine and pull it off at engine end, you can turn the hose as the 90deg check valve will rotate, so end of hose is closer to fender
Blow into the hose, you shouldn't be able to, that means Check Valve is working
Suck on the hose, that's a vacuum, it should HOLD that vacuum, if you can continue to suck on the hose then booster is leaking internally, you might even hear the leak

 

[Jazzer]

Use a turkey baster for those tests, so you don’ t feel weird about it later....

-Jazzer

 

[Dirtman]

Use a turkey baster for those tests, so you don’ t feel weird about it later....

-Jazzer

Ron always wants people to suck on hoses...

 

[Jazzer]

I’ve heard Canadians sometimes call each other “hosers”, is that what thats means?

 

[RonD]

Yes, suck on the hose, you hoser, lol

In Canada Hoser means a "loser" and maybe stupid

I prefer to think it came from the Losing Hockey team having to "hose down" the ice so it was smooth for the next game, so the hoser was the loser

But some say it came from someone who siphoned/stole fuel with a hose

But its a Canadian expression, and theives siphon fuel every where not just in Canada

 

[helpme]

Thanks guys. That check valve is a $5 part. I thought that it was just a right angle connector. I'll pick one up in the morning.