Have read a few posts on here about the brake pedal going to the floor when the engine is running. I have some pedal resistance when the engine is off. As soon as engine starts, brake pedal goes to the floor. There are no visible external brake fluid leaks. All the threads I read said to start by bench bleeding the master cylinder. This car has ABS which I believe is first bled manually after the MC had been bench bled and reinstalled, and then again using a scan tool after everything is reassembled. If anyone can provide the steps in this repair, I would be most appreciative. The car has sat for 18 years now, so I suspect an internal MC leak. Just recently got the engine running. Now I have to get the stopping part down.

Today I tried pumping the brakes while the engine was idling, and noted that 5 or 6 quick pumps of the pedal would sometimes yield some brake pressure. It also seemed that the idle was stumbling sometimes when I pumped the brake pedal.

I've never taken a master cylinder out, bench bled it, then put it back in. If it is so far gone that it ever had a problem then it needs to be replaced.

After that, I bet this issue is solved.

Yes, if you are pumping the pedal a lot, the vacuum booster can cause that, especially if an engine has other issues.

I would suggest a new booster because it sounds like a bad diaphram. I would also replace the check valve and line to it. The master cylinder, I would suggest replacement if the fluid has evidence of being rusty or debris in it.

Put the booster under vacuum. Does it hold? Yes is good.

Manual Bleeding

MANUAL BLEEDING

NOTICE: Use only DOT 3 brake fluid from a clean, sealed container. Do not use fluid from an open container that may be contaminated with water. DO NOT USE DOT 5 BRAKE FLUID.

IMPORTANT: In the following steps, use a suitable container and/or shop cloths to catch fluid and prevent it from contacting any painted surfaces.

CLEAN
^ Fluid reservoir cover and surrounding area.

REMOVE OR DISCONNECT
^ Fluid reservoir cover.

INSPECT
^ Fluid level in reservoir and fill to the correct level if necessary.

INSTALL OR CONNECT
^ Fluid reservoir cover.

HYDRAULIC MODULATOR/MASTER CYLINDER ASSEMBLY
1. Prime hydraulic modulator/master cylinder assembly.
^ Attach bleeder hose to rearward bleeder valve and submerge opposite hose end in clean container partially filled with brake fluid.
^ Slowly open rearward bleeder valve 1/2 to 3/4 turns.
^ Depress brake pedal and hold until fluid begins to flow.
^ Close valve and release brake pedal. Repeat Step 1 until no air bubbles are present.
^ Repeat procedure for forward bleeder valve until fluids begins to flow.

IMPORTANT: Once fluid is seen to flow from both modulator bleeder valves, the hydraulic modulator/master cylinder assembly is sufficiently full of fluid. However, it may not be completely purged of air. At this point move to the wheel brakes and bleed them. This ensures that the lowest points in the system are completely free of air and then the hydraulic modulator/master cylinder assembly can be purged of any remaining air.

REMOVE OR DISCONNECT
^ Fluid reservoir cover.

INSPECT
^ Fluid level in reservoir and fill to the correct level if necessary.

INSTALL OR CONNECT
^ Fluid reservoir cover.

2. Raise vehicle and suitably support.

IMPORTANT
^ Bleed sequence:
- Right rear.
- Left rear.
- Right front.
- Left front.

3. Bleed wheel brakes in sequence shown above.
^ Attach bleeder hose to bleeder valve and submerge opposite hose in clean container partially filled with brake fluid.
^ Open bleeder valve.
^ Slowly depress brake pedal.
^ Close valve and slowly relase brake pedal.
^ Wait 5 seconds.
4. Repeat Step 3, including the five seconds wait until brake pedal feels firm at half travel and no air bubbles are observed in the bleeder hose.
5. Repeat Steps 3 and 4 on left rear wheel brake.
6. Repeat Steps 3 and 4 on front wheel brakes.
^ To assist in freeing entrapped air, tap lightly on caliper casting with a rubber mallet.
7. Lower vehicle.

REMOVE OR DISCONNECT
^ Fluid reservoir cover.

INSPECT
^ Fluid level in reservoir and fill to the correct level if necessary.

INSTALL OR CONNECT
^ Fluid reservoir cover.

8. Bleed the ABS hydraulic modulator/master cylinder assembly.
^ Attach bleeder hose to rearward bleeder valve and submerge opposite end in clean container partially filled with brake fluid.
^ Depress brake pedal with moderate force.
^ Slowly open rearward bleeder valve 1/2 to 3/4 turns and allow fluid to flow.
^ Close valve and release brake pedal.
^ Wait five seconds.
^ Repeat above steps, including the five second wait, until all the air is purged from the system.
^ Repeat procedure for forward bleeder valve until all air is purged from the system.

REMOVE OR DISCONNECT
^ Fluid reservoir cover.

INSPECT
^ Fluid level in reservoir and fill to the correct level if necessary.

INSTALL OR CONNECT
^ Fluid reservoir cover.

INSPECT
^ With ignition "ON" apply brake pedal with moderate force and hold. Note pedal travel and feel.
^ If pedal feels firm and constant and pedal travel is not excessive, start the engine. With the engine running, recheck the pedal travel.If it'* still firm and constant and pedal travel is not excessive, go to Step 9.
^ If pedal feels soft or has excessive travel either initially or after engine start, use the Tech 1, "Release" then "Apply" the motors 2 to 3 times and cycle the solenoids 5 to 10 times. Be sure to "Apply" the front and rear motors to ensure the pistons are in the up most position. Now repeat the procedure and start at Step 1.
9. Road test the vehicle.
^ Make several normal (non-ABS) stops from a moderate speed to ensure proper brake system function.

pro multis · About Operation CHARM

[QUOTE=carfixer007;1648571]Put the booster under vacuum. Does it hold? Yes is good.

Manual Bleeding

MANUAL BLEEDING
[QUOTE]

Thank you very much. Just what I was looking for.

At Rockauto:

ACDelco 18M669 is $100.79
DORMAN M390276 is $66.89
DYNAMIC FRICTION 35547075 is $54.99

I would trust any of these a hundred times over a 30+ year old master cylinder that has issues.

Just sayin'.

Posting a follow-up.


The initial problem was that I had brake pedal until I started the engine, then the pedal would go to the floor. I did not have any evident external brake system leaks before beginning the repair. A new master cylinder took care of my issue.


The instructions posted by carfixer007 were helpful. By reading them I learned the name by which the ABS device is known, which is "hydraulic modulator." I googled that term and learned that this car came equipped with the Delco ABS-VI system. Googling that term led me to this article about the ABS-VI system: https://www.j-car.org/forums/read.php?f=3&t=137165&a=1


This was a very helpful article. It explained why I was at first unable to bleed the rear brakes after reinstalling the master cylinder/hydraulic modulator assembly. It was because the rear pistons in the hydraulic modulator were not in the "home" position. I did not have a scan tool to reset the pistons to the home position, so I followed the instructions in the article to manually reset the pistons. I was then able to bleed the rear wheel cylinders and brake lines.


The master cylinder kit that I bought did not come with everything needed to do this repair. The article to which I previously referred says to use new connector tubes with new o-rings when connecting the master cylinder to the hydraulic modulator assembly. My kit did not come with these. It did not even come with o-rings for the tubes. Additionally, when bench bleeding, you must first connect the master cylinder to the hydraulic modulator assembly, then bench bleed until brake fluid is coming from the brake line ports of the hydraulic modulator assembly. There are four ports, but two are a smaller fine thread and two are a larger course thread. The m/c kit I bought had bleed adapters for the two smaller fine thread ports, but not the larger course thread ports. I found a couple of rubber plugs to seal the larger ports when I bench bled, and was able to get fluid flowing through all four ports without making a huge mess. But if you wanted to do it right, you'd need bleed adapters for all four brake line ports.


While researching this issue, I came across a yt video in which a mechanic had this same problem, but a new m/c had not fixed it. In that video, the culprit was a rear wheel cylinder that was not leaking, but was sucking air when the pistons returned after brakes were applied. He replaced both rear wheel cylinders and that resolved his issue.

Thanks for letting us know how it turned out!