Had something really odd happen today. I drove my truck from Kent, to Seattle, to Redmond (about 35 miles). Everything was norrmal. I then drove it back from Redmond to Seattle (15 miles) and everything was totally normal until I pulled off the freeway off ramp. When I hit the brakes to slow down the pedal was REALLY firm and the braking was just about nil.

I'm a big guy with really strong legs so I was able to put enough stomp on it to get the brakes to stop the truck, but it surprised and even scared me a little. There had been absolutley NO indication of any problem untill suddenly I had almost no brakes. Driving it the couple of miles from the freeway offramp to my office I applied the brakes a few times to see if anything changed. In the process I determined that the rear brakes seem to be working, but the front brakes don't seem to be doing much of anything. It takes a pretty hard push on the pedal, and the rears will lock up while the fronts don't seem to be braking at all.

What the heck could cause it to do that all of the sudden? Unfortunately I'm making a business trip for the next two days and leaving my truck at the office (driving a rental), so I won't be able to check anything on it until I get it home Saturday. In the meantime, any and all theories and diagnostic suggestions are welcome. This is my DD right now, so I need to get it fixed pretty quick on Saturday if at all possible......

 

vacuum pump or power booster took a dump.

 

Kinda what I was thinking too. Is there a hose directly connected between the pump and the booster? If so, I guess the first step would be unhooking it at one end or the other and seeing if there's vacuum from the pump. Any other suggestions?

 

yup. one big ol hose almost straight from the pump to the booster.

at least on the 88to91 trucks i had..pull the pump off the booster, and fire the truck up. should be at least 18 inch of vacuum. that should be enough to almost not be able to pull your hand off the hose. if you have a good vacuum in the hose at the booster, hook it up. run the truck for a minute or two. then shut the truck off, and pop the hose off the booster. there should be a one way valve in the hose end to keep vacuum in the booster. if you hear a whoosh when you pull the hose the booster is most likely good. if you don't, the booster diaphram ripped and is not holding vacuum.

 

just curious about something. which truck?? the 4 speed, or the C6?
if it is the C6 truck, when you loose the vacuum pump you will also loose proper trans function.meaning it will shift real funky.

 

i would disconnect the hose from the booster and feel if it is pulling a vacum also lossen the belt and turn the pump over by hand to make sure it is spinning okay.

 

did the brake light come on? last pump i changed out when it was bad the brake light came on.

 

Definatly sounds like something failed in the brake booster system (not a biggie). tjc, you beat me to it. I've had two alternator belts fail on me while cruising on the freeway (every now and then I still find pieces). Since the vacuum pump is driven off of the alternator, all vacuum related systems will be affected, cruise control, auto trans, some climate controls, and of course, power brakes. As for the auto trans, it will upshift as though you are flooring it all the time when accellerating, and downshift as though you are using engine brakes.

 

GOOD POINT! It is the C6 - the 4 speed isn't ready for DD duty yet, and probably won't be 'til summer.

The tranny seemed to be shifting normally. That makes me think the pump is working and it is the booster that's the culprit. Would it act that way if it was just the check valve in the booster? Would the rears work a lot more/better than the fronts if that's all it was? I know the fronts being disks require moving a lot more fluid than the rear drums to work.

That kind of had me wondering if it might be something to do with the proportioning system, or even the master cylinder. But if it was the master cylinder, the pedal would be softer - not harder - right?

On the other hand, since the fronts DO require pushing more fluid to work, not having the booster to assist might just make them pretty much inoperable. That kind of makes sense. The fact that the brake warning light never came on also makes me think the pump is still working.

Think Napa is likely to have the booster in stock?

 

mine has done it 2 tice since i have been driving it, so i feel i need to change the vacuum pump some time in the future

 

in stock, no. but it should not take them more that a day to get it from the warehouse.
with a properly working trans, i think it may just be the booster.
but to be sure, check for vacuum anyway, and check the booster to see if it is holding vacuum. like i said, when you pull the hose out of it, you will hear a "whoosh" as the booster sucks air in. if it don't you gots problems.

 

If the back brakes are working better than the front, then that tells me that either your mastersylinder is shot, or the proportioning valve is stuck. I drove my truck around for awhile with no vacuum pump, while i waswaiting on a new one to come in the mail. I never had to push that hard to get it to stop.Even with no boost, it wasnt that hard. I would check master cylinder

 

Well, I found the problem. I got back from my business trip last night and returned the rental car this morning. My wife drove me from the rental car location back to my office to pick up the truck. When I went to start it it cranked really s-l-o-w-l-y.

Went to jump start it off my wife's Jeep and saw the problem as soon as I popped the hood. The alternator belt had totally disintegrated. No alternator drive belt = no vacuum pump drive since it is driven off the double pulley of the alternator. No alternator also resulted in draining the batteries.
Swung over to the local Napa and picked up a new alternator belt and installed it while the Jeep was hooked up and charging the truck's batteries. Fired it up and all is well - brakes working normally again.

The best I can figure the belt came apart while I was on the freeway and since it was tooling along in 3rd gear and didn't have to shift until I got off the freeway, the couple of miles between the off ramp and the office at 30 mph wasn't far enough to require enough shifts for me to notice anything odd.

So all is well and the repairs only cost $20 and 20 minutes. Thanks to all for the advice and suggestions. If I hadn't been in such a hurry that I didn't have time to even look at it before posting I wouldn't have even had to ask (obviously), but I probably would have posted just to tell about the problem anyway

 

That happend to me twice so far, I can't seem to get an alternator belt to last more than a year. Ths silly thing is so darn long and likes to flap alot.

 

Always get premium grade belts for the alternator, AC and power steering belts. They are all so long they like to slap, the premiums seem to slap a lot less.