On my '93 f250, a couple times a day, the brake pedal gets hard like there is no boost. The truck stops fine, but it makes my girlfriend a little nervous when I go to brake in traffic and I say "oops, no brakes!" After having a hard pedal, the next time I brake (and sometimes the rest of the day) it is fine. I pulled the hose that goes to the vacuum pump and it is about 19" steady at idle (@ 5000' elevation). I also changed the booster check valve. I am having trouble trying to figure this out, as it only happens once in a while. It seems like it may happen more while I'm backing up (in parking lots etc) . I looked over the vacuum lines in the engine compartment and they don't seem to be cracked or deteriorated. I'm hoping you guys have a suggestion, otherwise I guess it will drive it until something breaks (leaving a little extra room in traffic, and getting an occasional right leg workout).

 

Seizure,
If I recall the vacuum is supposed to be 23". That may be different at the 5,000 foot elevation, not sure. Did you press the brake pedal while you had the gauge connected? How quickly did the vacuum build back up? How long will it hold the 19" after shut down?

My guess is a bad/going bad vacuum pump but without some further testing it's hard to say.

Mike

 

I have this exact same problem on my e350 with the 6.9 diesel. The hard pedal issue only happens when it is running at low RPMS. Coasting up to stop signs, parking lots etc.

I suppose it could be as simple as a loose belt but I havnt looked into it yet. This quadravan is a bitch to work on. I guess thats one of the drawbacks of a van. :-(

If anyone knows whats causing this let me know I'd really like to get it fixed. Thanks!

 

There is a vacuum test available........if there is low vacuum after pedal use and delayed recovery then the pump needs to be replaced.........

 

I did the vacuum test. When I put in the t between the pump and brake booster, I got some strange results. If I pushed hard on the pedal the vacuum dropped from 19 a few pounds and held. If I pushed lightly or medium, the vacuum would drop substantially and the needle would quiver. Repeated light pumpings made the vacuum drop to down to about 2".I got a new booster and hopefully will have time to put it on in the next couple of days. I will repost when I get it in and let you guys know if that worked. I am guessing there is a worn spot in the booster where I do the most braking that causes a vacuum leak in a certain section of pedal stroke (the most frequently used -light to medium) and maybe the draw down of vacuum after frequent like braking is why the pedal gets hard more often when parking than on the highway. But then again, maybe I am making all this reasoning up just to find an excuse to buy the cheaper booster than the more expensive vacuum pump. Thanks PLC7.3 for the link to the vacuum test!

 

As a follow up, I replaced the booster first, and still had the intermittent brake problem. Changing the vacuum pump fixed it. The brakes work MUCH better now. The pedal doesn't feel as spongy and doesn't sink as far. The parking brake feels better. I am guessing that rod that stuck out of the old booster, into the master cylinder was adjusted out too far compared to the new one. The rear brakes self adjusters seem to work now as well. You guys have the best advice. thanks again.

 

Thanks for the info. I just took my rig out and the first time I came to a stop that brake issue got my attention. hehe. Really gotta slam on that thing when you don't have the power assist!

 

Same Problem Here Except You Need To Change The R.a.b.s Vavle, I Spent Alot Of Time And Money, Replaced Everything I Mean Everything...... Even Took It To Just Brakes 3,times They Told Me Rabs Valve. I Replaced It And Stop On Adime Now... Jim

 

What is the R.A.B.S valve?

 

the rabs valve is the Rear Antilock Brake Valve. I had already changed a year ago when trying to get the pedal to not sink so far when braking and not having luck bleeding the brakes. I'm not sure about this, but in my case, I think the problem might have been that the rod in the booster was sticking out too far (into the master cyl) so the master cylinder may not have been getting enough stroke on the rear brake circuit. Of course, as you guys may have figured out by now, my technique of fixing the truck is to try to figure out what is wrong and then guess like a monkey what part is bad, replace it, and if that doesn't fix it guess again. Not exactly the cheapest way when it comes to parts, but very cheap on labor. Eventually, if I keep working on my truck like this, I will have all new parts!

 

85 will not have a rabs valve look on the rear axle and see if the load device is still in place this is a valve with a rod it helps when new this adjust the breaks to the amount of weight is on the truck

 

I am doubting that was the problem. If the rod is too long and pushing too much on the master cylinder, the usual symptoms are after driving a few miles the brakes start dragging.

The two little holes in the bottom of the brake resevoir are wide open to the front and rear brake lines. These are called compensation ports. As the brake fluid in the lines and calipers warms up, it expands, and goes through the compensation ports back into the resevoir. When you apply the brakes, the compensation ports are blocked, and the piston in the master cylinder builds pressure.

If the master cylinder rod is adjusted too tight, these compensation ports can be blocked all the time. So when the fluid gets hot, instead of relieving itself back up into the resevoir, it builds pressure and starts applying the brakes.