Brakes problem in 88 7.3L: Master cylinder or vacuum pump??
 

In my 88 7.3L, the brakes just pretty much went to crap while driving on the highway, I think somehow triggered by having to drive on rumble strip for a good 2 miles at 55mph due to construction. Pedal became really soft, then eventually became hard, and sometimes goes back to soft, but the truck never stops like it used to.

I was wondering if someone could tell me how to tell if it's the master cylinder or the the vacuum pump. I have a feeling it's the vacuum pump, because I can brake better at highway speed, due to the pump getting enough power to feed the brakes, if my understanding of the system is correct. Let me know if this is a correct assumption. I NEED BRAKES!!

Thanks.

Eric

I have the same exact problem. I just posted a thread a couple days ago and Dave told me the vacuum pump was crap but when I checked it at idle it had a good vacuum. I am going to replace the power booster to see if it helps. I first tried replacing the master cylinder but that did nothing. When the pedal gets stiff it is near impossible to stop! Let me know if you figure something out. I cant drive without the brakes.

How do I check the vacuum?

The vaccum pump is also the lift fuel pump. There is a fairly large (~1") black hose that runs from the pump to the break booster. When I checked my vacuum pressure I just unhooked the hose from the booster unit. If you start the truck and put your finger/thumb over then end of the hose it should be sucking fairly hard. If you have or can get a vacuum gauge the pump should be pulling at least 7 to 10 inHg if I recall (someone correct me if those arn't the right numbers)

Hope that helps

Start with cheapest fix
 

Hey

Had same problem with mine. Started with replacing the vaccuum lines. Really cheap. I suspect it is your vaccuum pump because that is exactly how mine reacted when my vaccuum pump went. Anyway start with the cheap things first

jeff

when the vacuum pump is bad , the brake pedal is hard and the blower motor work only in defroster position.

The vacuum pump is seperate from the fuel lift pump. The vacuum pump is located right above it.
So you guys are saying if I replace the booster I am wasting my time and money? I dont want to get the powerbooster on and have the same problem minus 75$.

If you brake pedal is really hard to push then it is usually your vac pump. We have a fleet of the ford f-350 88-90 and this seems to be a common problem with them. Try replacing the vac pump and i will bet that will solve your problem.

Sorry to jump on your thread ericolicious, but I have a similar problem. At slow speed or until I get a few miles on my 92, the pedal is rock hard. After that brakes feel better, but not quite normal. Problem occurred suddenly. Bam it was there. Vacuum pump still? Thanks.

still sounds like it might be the vac pump possibly the pulley slips and then when it catches then you get vacuum. i have seen this before. How many miles are on the truck? have you ever replaced it before? when the brakes feel normal when you step on them do they grab and kinda give you whiplash? if they grab or lock up it could be your R.A.B.S. valve.

Cool, yes 215,000 miles. Dont know when replaced as I have only had the truck for a year. I have noticed too that the rears do seem to lock up over the last 2 weeks more than they should, but no, the brakes are not jerky or unpredictable. I thought the locking up may be an adjustment issue. I will look into the vac pump and adjustment first. Thanks.

yeah the rabs valve isnt cheap i think the last one i got from ford at our fleet agreement price was 367.00 so normal price probably would be 450 so dont replace unless you have to