1998 GMC Sierra Brake Bleeding? (ABS Question)

[MBood82]

Any suggestions for bleeding the brakes on a 1998 GMC Sierra? It has the 4wheel ABS and we replaced some brake hose, the rear wheel cylinders, the front calipers and the master cylinder. Brakes work but are mushy. Is the ABS module involved at all?

Additionally, is it normal for the vacuum booster to flex a bit?

[xj_man_646]

Bleed it like every other vehicle? I've never had trouble with an abs rig, OBS chevs included

[xj_man_646]

How much fluid did you pump through?

[MBood82]

Originally Posted by xj_man_646

Bleed it like every other vehicle? I've never had trouble with an abs rig, OBS chevs included

That's what we did, it's just got a spongy pedal still. It doesn't bottom out and will engage the brakes hard enough to lock up the tires (or at least engage the ABS) but it doesn't feel right.

Originally Posted by xj_man_646

How much fluid did you pump through?

That depends, do we include the amount that we ejected out of the resevior every time we forgot to put the cover back on? My helper hadn't seen that before so he forgot to put it on and I forgot to check myself so we had a number of times that it ejaculated all over the engine compartment. I think we bled through, both by gravity and pumping, more than a bottle and a half. Another bottle and a half is covering my engine compartment and hood.

I'm retarded sometimes. Not Kyle retarded but not smaht, either.

[wrath]

If you've only put 3 pints through, that's not nearly enough. It will probably take a quart just to do the rears and that's with competent help.

[MBood82]

we were using the big bottles, not the small ones. I'm terrible with units of measure though so I have no idea if they are pints or quarts. I suspect quarts.

What is it that makes it take such a high volume? We had no bubbles coming out the lines a long time before we finally gave up.

[wrath]

Need to go fast. Meaning someone presses on the pedal hard, you open the bleeder then close it before they hit bottom. Rinse, repeat, faster than the air can go back to the high spot in the line.

1.5 quarts (3 pints) is not a lot of fluid in a system with modern ABS (has a pump in it). It's the reason pressurized bleeders were created (faster, less waste).

[xj_man_646]

spongy pedals are also nature of OBS chevys as far as I'm concerned, especially vacuum boosted ones. I can't tell you how many times I've thought that the pedal seems spongy, but the brakes all work fine. Depending on how spongy it is, it could be 'normal'

[xj_man_646]

I assume you went Passenger rear, driver rear, passenger front, driver front, for the bleeding sequence? If not, I'd re-do it, in that order, making sure that you are getting good clean fluid out of each bleeder along the way. I might even go as far to say that I would bleed each corner until 1/2 the master cylinder had been pumped through, or more if the fluid continues to look dirty.

[wrath]

It takes half a pint of fluid just to fill the line from the master to the right rear, not including the wheel cylinder and the valving/passageways in the ABS/combination valve.

Seems like on those trucks the ABS unit was on the driver's wheelhouse so it could be a chore to get any air out of it. The newer trucks aren't too bad because the unit is in front of the fuel tank.

[MBood82]

Originally Posted by wrath

Need to go fast. Meaning someone presses on the pedal hard, you open the bleeder then close it before they hit bottom. Rinse, repeat, faster than the air can go back to the high spot in the line.

1.5 quarts (3 pints) is not a lot of fluid in a system with modern ABS (has a pump in it). It's the reason pressurized bleeders were created (faster, less waste).

I'll look into having a professional do it, we were definitely not doing it fast enough before and for what it would cost to have a professional bleed it it's probably worth it. All the other work has been done.

Originally Posted by xj_man_646

spongy pedals are also nature of OBS chevys as far as I'm concerned, especially vacuum boosted ones. I can't tell you how many times I've thought that the pedal seems spongy, but the brakes all work fine. Depending on how spongy it is, it could be 'normal'

I do agree with that but I am pretty sure it wasn't this spongy before we began. It's hard to tell for sure though, we fiddled with it so much and I drove the Magnum with a rock hard brake pedal in between so it might just be bad memory. I do have to put it down pretty far though to get any response so I do think it would benefit from more bleeding.

Originally Posted by xj_man_646

I assume you went Passenger rear, driver rear, passenger front, driver front, for the bleeding sequence? If not, I'd re-do it, in that order, making sure that you are getting good clean fluid out of each bleeder along the way. I might even go as far to say that I would bleed each corner until 1/2 the master cylinder had been pumped through, or more if the fluid continues to look dirty.

Yes, we definitely did it in the right order.

Originally Posted by wrath

It takes half a pint of fluid just to fill the line from the master to the right rear, not including the wheel cylinder and the valving/passageways in the ABS/combination valve.

Seems like on those trucks the ABS unit was on the driver's wheelhouse so it could be a chore to get any air out of it. The newer trucks aren't too bad because the unit is in front of the fuel tank.

We didn't let the brake lines completely drain so I doubt we needed that much volume for that reason but I would blame inadequate pumping techniques.

It is located on the inside front fender on the drivers side, it's below the master cylinder but the lines dip below the master before they meet the ABS module so it is possible for air to be trapped and not be interested in evacuating.

[xj_man_646]

maybe try renting a power brake bleeder?

[MBood82]

I'll see what is local, I figure the chances of me screwing it up worse and being out the cost of a power bleeder versus paying a local guy a half hour of labor might lean it in the favor of paying someone.

[fuzzy]

i know it's late, but on my 90 I had to break down and get a hand pump vaccum pump to bleed them... after what seemed like hours it finally went back to the brake pedal I had known and loved.

there is something to be said for those little hand pumps.

[MBood82]

I've been keeping my eye open for one to buy/borrow but haven't looked too hard really... the brakes are functional right now, just a little spongy. Everything I have read online though at this point indicates that you really can't effectively bleed these brakes without a forced bleeding device of some sort.

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[MBood82]

cool, thanks for the info