new guy with a question

[heff]

hi, new to the forum, but been lurking a while now.

I have a brake problem, and i need some guidance.
Engine off, super hard brake pedal.
Engine running, pedal will go to floor, sometimes. Then the brake light comes on. But this
is only sometimes, mostly at idle. On the highway, or around town, steady rpm, no issues.
Really good brakes actually. But i want to fix this before any thing surprising happens.

no loss of fluid, anywhere, yet, that i can see. There was a drop where to master cylinder bolts
to the booster, but repeated engine on/engine off pedal pumping/holding did not create any
more errant brake fluid. No wheel cylinder leaks, no abs light. I replaced the front soft lines
when i got the truck. The master cylinder and booster were replaced a week before i got
the truck. From what i've read this sound like a booster issue, but i just wanted to be sure
i was heading in the right direction.



Thanks in advance

 

[flareside_thun]

I'd say either booster or a weak vacuum pump.

 

[towcat]

do you have ABS brakes in the rear?

 

[heff]

yea the vacuum pump. i noticed when the brake pedal is pushed down hard the vac pump seems to make an odd noise.
That's the reason i asked actually, but forgot to add info that could have been useful. There isn't any hissing or anything like that,
just a noise i haven't noticed, but then again, i haven't had someone hold the pedal with the hood up either.
I did replace the belt with a nice, sticky rubber bando belt, to replace the gates that showed signs of slipping. maybe the belt actually
making contact to the vac pump pulley made it show it's age? i suppose i should check the vac pump anyway

and yes, rear wheel abs. but like i said, no trouble codes, no abs light, no weird brake lock-up. only light that has came on is the " ((brake)) " light

 

['94IDITurbo7.3]

need to know EXACTLY what the vac pump is doing and when. hook up a vac gauge and go for a drive. i bought a cheap sunpro mechanical vac/boost gauge from a-zone and ran the vac line from the little vacuum manifold on the drivers side fender into the cab and to the gauge. now i know what is going on with my vac at all times.

welcome aboard by the way!

 

[towcat]

isolate vac pump first. if no solution, unplug the ABS unit at the frame and see if there is any change. often when the ABS fails on these trucks, the ABS does not throw codes at all. the dump valve leaks and you lose pressure and pedal will sink.

 

[heff]

how do i isolate the vac pump? i mean, how many inches of vacuum should i be pulling? i assume the needle should stay steady?

pulled abs fuses, abs light was flashing on dash. it's getting dark so that will put an end to much more trouble shooting, but i did notice
when the womern was pushing/pumping the pedal i could put my hand over the vac pump and i could feel air coming from the holes on
top, i guess what covers a diaphragm. the pedal was firm until she repeatedly pumped the pedal, then the vac pump would stat to make noise
and i could feel the air coming out of the holes.

i'll put a gauge on it tomorrow and see what that does

 

['94IDITurbo7.3]

i think perfect vac is a touch under 30inhg, my gauges stays RIGHT under 30inhg all the time unless i am using the brakes.

yeah you want a steady needle, when the engine is running you should be able to pump the brakes real quick a couple times and the vac should not go way down, then it should build back up fairly quick.

i have some vids on my phone of me doing some vac diag work on my old pump. i should see if i can post them up. that would show you how a bad/failing vac pump works.

to isolate the vac pump from the system, there is a rubber tube coming off the bottom of the pump, follow that tube to a manifold type set-up on the drivers side fender, pull the tube coming from the pump off that manifold, then find some kind of adapter to go from your vac gauge to that tube. then start the truck up. that will tell you what the vac pump is putting out.

 

[heff]

when i hook up a vac gauge, i'm getting about 11inhg. brakes applied, vac drops quickly down to about 7-8in, second pump goes down to about 5.
builds back up to about 10-11in, then holds steady. that seems pretty low to me, especially if around 30in is the norm

So..after driving and engine at operating temp i have 21in steady vac. apply brakes, needle doesn't really move, but i can still push the pedal down further than i feel it should go. hold brakes at a stop sign, let up, and hold again causes a sudden drop in vacuum, with a slowish build up. No red warning light this time.
any ideas? rabs or vacuum pump? or am i just making a big deal out of nothing?

 

[DragRag]

Disconnect the brake booster from the vacuum lines, plug line to make sure it's a sealed system still. In other words eliminate any other leaks to. Check vacuum running now. If it is still low, your pump is likely bad. There is also a chance your break booster could have a big leak to, and you would never achieve anywhere near 29.99 inches of HG. Also, if you have ANY brake fluid coming out of the back side of the master cylinder it is bad. Good luck.

 

[heff]

i plugged all the vac lines on the vacuum manifold. no changes, held steady at 21inhg as before. so no changes with anything hooked to the vacuum manifold should indicate no losses of vacuum, and makes me think the pump itself is taking a dump. guess i should replace that pretty soon then.

 

[icanfixall]

21 inches of vacuum is kinda low to keep the brakes working properly. Might as well replace the vacuum pump but... you will need the special pulley remover installer to do that. The puller grips the lip right at the shaft hole in the pulley. If you try to remove the pulley by grabbing the belt diameter it will ruin the pulley. Yes, Ford still sells these pulleys. Most fake autoparts stores will rent the puller.