Bleed / cycle ABS pump?

Hey guys

Just picked up neglected '99... brakes only barely work. I don't know much history, but I suspect the reservoir was allowed to run dry and was refilled without bleeding. I say this because even though I haven't bled at the wheels, I do know the brakes were professionally done some years back but have not been touched since. So I think it may have sucked air at the MC. Pads and rotors are OK. Pedal is soft but can be pumped up. I saw the write-up about bench bleeding the reservoir, but my question is this.... do I not have to do anything to cycle the ABS pump while bleeding? I thought all GMs were that way, but that is based on limited experience.

Thanks

Dave

I'm assuming that the pedal goes to the floor?

Yes

One more question.....

Does it make any type of "pop" sound, or get any resistance in the pedal before it goes to the floor? Or is it just a consistent "fall" to the floor?

Definitely resistance on the way down. Not sure about pop. Have only been able to test it in my condo parking lot (will have garage again in June... thank god.)
Forgot to mention-- I pulled off both left wheels to inspect brakes. Pad/rotors OK as mentioned. I tried compressing the piston on the front caliper... took a lot of initial pressure, but it suddenly popped free and compressed easy after that. I saw that the rear caliper is the screw-in piston type-- couldn't try it at the time because my wife was out with my truck where my rear caliper "Borg cube" is stored.

When I put the front caliper back on (compressed), I tried pumping up the brakes to re-seat the piston. Engine off so no power. Took a LOT of pumping just to seat the piston, but it eventually did.

Also forgot to mention that the car sat for 4 months before I bought it and had it towed home.

Realistically, you'd want the ability to command the different ABS channels ON/OFF while you bleed the brakes. This can only be done with a Tech II or equivalent. If you had access to a Power Bleeder, the same effect can be had. All it does is force the fluid through everything. If you suspect air in the lines, you'd want to start right at the lines on the MC, then work down to the ABS pump, then the calipers.....

The reason I asked those questions was to try and figure out of the MC is bad or not. More often than not, you will still have some brakes, but no pedal feel. The MC piston seals, shaped like little cups, can roll over when pressure is applied, causing the mush. You may or may not have the strength to make this happen without the car running. I've put an MC on mine for this issue.

If you don't drive it, I would try gravity bleeding it one wheel at a time. Just make sure the MC is full, and you leave the cap off. Or, if you have help, just pump/bleed until the help is blue in the face :lol
; Then and only then, if you cannot get the pedal back, I might be throw an MC on it.

If the MC is run dry, air might have entered the ABS unit. I don't know this from personal experience, but have read that any air must be purged by cycling the valves in the ABS unit + applied vacuum at the bleeder.

If you haven't done the pads yet, might try compressing the caliper pistons, which will push fluid and air back upstream, possibly getting removing some air through the MC.

From there, using a mighty vac to bleed all lines from the bleeder should remove most of the air. If not, may need to have the ABS unit cycled.

Many but not all mechanics have a 3rd party device that will cycle the ABS.

Power bleeding in my experience gets the air out of the ABS - BUT - don't take my word for it. That said, I've used a power bleeder both on the Riv and a Lincoln I used to own, with great results.  On the Riv, I had the brake system apart for 150,000 and 300,000 mile maintenance (each time an overhaul: replace calipers, MC, soft lines) and on the Lincoln I power bled because a Ford dealer (now out of business) replaced a caliper and gravity bled the system, which due to the way the brake system is installed on the '95 Continental *does not work* - IMHO don't ever gravity bleed that system.  

There are instructions for building a power bleeder on this site (somewhere).