I am having a problem with the brakes - they dont stop the truck!!
It seems as if the "power assist" may not be working. The pedal seems spongy. The brake fluid is black!!! Front calipers seem to be working OK - no sign of any leaks at the calipers or elsewhere.
Could it be a problem with the vacuum assist unit? How do you check it? Could it be the front flexible hoses breaking up inside - they are original - truck has 102,000miles on it - hoses are original.
When you press the pedal it appears to go all the way to a solid stop. Even hard down the wheels will not lock on dry pavement - no sign of the ABS working. No ABS warning light.
A bad power booster will cause the pedal to be VERY hard to push. To check it, with key off pump the pedal a few times then with pressure on the pedal start the truck. Your foot should go down an inch or two when you start it (helping you push the pedal) The vacuum line can be somewhat loose and turn in its hole as lone as it doesnt leak. Does it seam you pedal goes way to the floor when you brake?
I pumped the brakes with the engine off until the pedal was hard -perhaps three pumps. I then, whilst holding the pedal down started the truck - the pedal went soft and almost to the floor. I guess the booster works - where next - master cylinder by-passing fluid?
"Battleax" wrote in message news: snipped-for-privacy@magma.ca...
He's got a system the brake fluid hasn't been replaced in since day one, and a master cylinder that has taken a dump. Empty the master, turkey baster works, wipe it out clean, fill it, bleed the brakes till the fluid runs clear, and then replace the master cylinder, and bleed again. If the brake hose show signs of cracking or peeling, replace them, other wise don't worry about them. The only time you have to replace them for fluid is when some yahoo has put the wrong fluid in the system, you'll know because the rubber gasket on the master cap will be swelled about twice as big as it should be. The fluid gets black from moisture absorption(brake fluid is a hydroscopic fluid, and is not petroleum based), being over heated, and from small bits that wear off the rubber seals on the moving parts of the system, but mostly from moisture. Moisture content lowers boiling point, hence the warning to only use brake fluid from a sealed container. 2% moisture content can lower boiling point 30%. Do not use Silicon based Dot5 brake fluid in the system, the antilock brake system will cause it to foam, and you will loose the brake pedal. Personal opinion for what its worth. If the master is aluminum, stay away from rebuilts and buy new. There is a another possibility and this is if you have just rear wheel anti lock brakes the RAWBs valve has gone bad and this will cause extreme low pedal and hard stopping. However this was more a problem on early 90's ford trucks than on General Motors products.
I think I will bleed the whole system first to remove the old fluid. Then fit the new master cylinder and re-bleed the system. This should hopefully ensure a clean system.
Would it be worthwhile replacing the front flexible hoses at the same time?
The only thing that bothers me in this are the requirements for the ABS part of the system. Does it matter or does it bleed as I do each wheel?
It would be easier and just as good to change the master first, then bleed each wheel till you get clean fluid comming out. Probably no need to replace anything else unless you think there is a problem. As far as the ABS just leave it alone, act like its not there.
Remember you have to bench bleed the master before you install it or you will never get a good pedal. I like using plugs instead of lines or hoses looped into the reservoir. You just pump the master with a screw driver or wooden dowel while its on the bench in a vice till you cant push in any more. Using hose or lines, when you think its done, using the HANDLE of a small hammer, or handle of a screw driver tap on the bottom of the master and stroke it some more, repeat till no bubbles come up out of the compensating ports when you tap and its done.
You can also do it on the car.....pump it with everything hooked up, then bleed it like normal. Once you have a good pedal work your way around to each wheel. Make sure to keep fluid in the master all the while or you will have to start over.
If the master cylinder bore isn't close to level, you can pump till your leg falls off, and not get all the air out.. As to the plugs, the replacement will come with lousy cheap plastic fittings and a couple of too short rubber hoses to slip over the nipples on them. I've bought both metric thread and SAE thread flare nut fitting plugs at NAPA in the past. If the threads are SAE, you can usually get them at a good hardware store as well. Cost about a buck a piece, if that.
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