I'm working through the brakes and have narrowed my symptoms down. I have a new booster and completely rebuilt rear brakes all bled out. I am pulling 22 inHg out of the vacuum pump at idle and it doesn't really move when rev'd to 2000 rpm. At the brake booster I am only getting about 7 inHg at idle and then it jumps to about 16 at 2000 rpm. The line from the manifold to the booster is brand new. All other lines off the manifold are getting about 16 at idle.

When driving the pedal feels fine and the truck seems to stop well enough (though could be better) but I get a brake like almost every time I stop at a light which then disappears once I get going again.

I'm thinking the vacuum pump may not be able to keep up but why am I getting so little vacuum to the booster? Is there something in the vacuum manifold that could be busted?

Also, cruise control works fine and the A/C controls work.

Thanks.

 

Did you get a new booster check valve? Are you getting a hard pedal?
The light can be triggered by the propotioning valve too, but most likely its vaccum, no the pump should not only put out a given in of vaccum but it has to do so in a given amout of time.
But first put the vac guage on the booster and pull one line at a time and pulg the vac block and see if the vac at the booster rises, this would indicate a vac leak somewhere else, if doing that changes nothing check the pump on post #14
https://www.ford-trucks.com/forums/1...cuum-pump.html

Theres also some good info on checking the booster in there too.

 

look for leaks on all the lines and the vacuum canister. 22 at the pump tells me the pump is good.

 

If the pressure doesn't change at the pump but does at the booster when revving, there absolutely must be a leak; that's the only way those results are possible.

As 84 said, it is also a time (= flow rate) issue with the pump; it could make good pressure but not keep up in terms of flow. I know at least on the newer trucks, the dash light doesn't come on right away with low pressure, it comes on with low pressure for a set time (either 5 or 7 seconds, IIRC)

Try plugging all the other outlets on the manifold and testing the pressure at the booster again. Then you can narrow down leaks by sequentially reconnecting the lines.

 

Well I pulled the pump off yesterday intending to swap a new one in but couldn't get the pulley off so I reinstalled the old one today and at first it tested good at 21 inHg. Then I shut the truck down, started it back up and now the pump was only putting out 6 inHg. I'm thinking the pump is on its way out but only shows problems intermittently.

I can't find a pulley puller that will fit this pulley so as soon as the new pulley comes in I'll just drop the new pump in and all should be good.

Thanks for the help.

 

It takes the same puller used on PS pump pulleys, Autozone should have a loaner.

 

I have that one but the collar does not fit completely down under the lip on the pulley so it won't stay on there as I wrench down on the bolt. I've tried vice grips, channel locks, and a bfh, but the ps puller is just too big.

 

I had the same issue, I just put the 2 halves on as best it could and tapped the ring on with a small hammer as far as I could. Was it the right way? No, did it work yes!

 

i have a puller that i modified with a grinder to fit the vacuum pump pulley.

 

I just swapped a new pump in this morning after finally getting a new pulley delivered. What a difference that made. I can actually lock up the brakes now.

Thanks for the help.