hello all, I finally broke down and bought a vacuum gauge to see if my v-pump is going out. I hooked the gauge up to the port on the fire wall and started the truck. Needle jumps quickly to around 23hg and then reads anywhere from 20-25hg at idle and under load but once I hit the brakes it drops damn close to 0 and is slow to build back up.

Is my vacuum pump weak or should I be looking elsewhere ?

 

bad vacuum pump.

 

Thanks for the quick reply Tom. That's what I was leaning towards but just wanted to throw it out to the FTE community.

 

you can bypass the brake booster and play with the heater controls flipping them back and forth. if you loose vacuum that is a sure sign.
but the fact it takes a long time to recover pretty much tells me the pump is doa

 

I have zero issues with the hvac but then again I need to replace the blend door

 

I didn't think the booster actually demanded vacuum until you let off the pedal? My understanding of the theory of operation is that there is equal vacuum held on both sides of the booster diaphragm, then when the pedal is depressed it vents the firewall side drawing the diaphragm and connected brake pushrod towards the master. If the diaphragm is ripped, it'll vent all the vacuum and not give any boost. On a gas engine this will usually stall the engine or cause running problems as well.
Disconnect the line, and test the vacuum. Put your finger over the end of the tube, if the vacuum jumps up nearly instantly I'd say the pump is fine and the booster is bad.

 

That's something I'll have to check in the AM Ford_six, thank you.

 

FORD_SIX: I disconnected the brake booster and plugged the hose. Vacuum jumps up almost instantly to about 20-21hg. ??? thinking bad booster then ?

 

What do you think Tom ? Thanks for everyones help

 

disconnect a vacuum line and see how much it drops. a small line should only drop a very little bit. if it nose dives, and then takes a while to recover when the line is plugged back up, i would say pump.

 

But if it recovers quick then look towards the booster? Just seems odd since I can plug the booster line with my thumb and it recovers quickly.

 

i really can't say, because the booster on my truck is the one it came with. i am on my 4th vacuum pump though.
when the truck shifts funky or the brakes start to get hard, i just put a new pump on.

 

Dang it Tom, lol maybe I'll just throw a vacuum pump and a booster on it but I dont really want to do that.

 

i disconnected a smaller vac line. It still dropped but recovers quick once I plug the line. Still seems to take an awfully long time to recover vacuum once the brake pedal is depressed. Makes me think the booster is sticking open or something...

 

Curious to see what fixes you 86F350. I have the same thing going on with my 93 F250. When I push the brake the damper for the A/C will switch from the vent position to the defrost. It's clearly losing vacuum. I checked everywhere for leaks and changed out the vacuum check valve. I was going to swap out the brake booster but am wondering now if it's the vacuum pump based on what I read above.