Hello all. New to this forum. I am looking for a ABS Modulator for a MY87. The right front brake was locked and I have cleared all the lines made sure the caliper was not seized. Also the front left works fine. I have seen other threads here with the same issue. I can't find one anywhere not even on vette2vette's website. How are people fixing this or are they being bypassed? Thanks

 

The ABS system works by RELEASING the pressure, so I wouldn't think it would cause your problem.

One thing you could try is pulling the ABS fuse and see how that affects your situation.

From posts that I have seen, the most common cause of one caliper locking up is a collapsed hose at the caliper. The hose is actually two hoses, one inside the other. The inner hose collapses but you can't see it. A new hose is a lot cheaper than an ABS unit...

 

Need more detail . . .

You say the right front 'was' locked.

Has it locked again when you hit the pedal?

What makes you think the modulator is the problem?

These Bosch units were well designed (as old as they now are) and can not restrict flow. As said above ASB works by releasing pressure to the skidding wheel, not applying pressure. If the modulator is not working your ASB light should be on and you simply will have no ASB action when you stomp on the brakes (not that you'd know since you can't drive the car).

It can't hold a brake on no matter how dead it might be.

You say you have cleared all the lines. Does this mean they were blocked or just that you made sure they weren't?

If you can bleed the caliper ok then I'd assume no blockage but it is possible for crap to block the flow in only one direction, in your case being the return direction.

I'd suggest rechecking the caliper piston by making sure you can push it in easily when the bleeder is open (don't push it too far or you can wreck the seal).

If the pistons moves freely then make sure the caliper is not frozen on its guide pins? When the piston pushes the inside pad against the disc it causes the caliper body to move in the opposite direction applying pressure to the other pad. These pins can get very tight and seize.

Take the caliper off the mounting bracket and clean the pins with a wire brush - reassemble using proper high temp guide pin lube only.

If the calipers are floating nicely on the guide pins then check that the clearance between the power booster rod and master cylinder (MC) is not tight and holding pressure.

Check this by cracking the front brake line where it goes into the MC and see if the caliper releases?

If that releases the brake then your MC is holding pressure and I'd suggest removing it and checking that you have a tiny bit of clearance between the power booster rod and the back of the MC piston. You'll see a little adjuster on the end of power booster rod when you remove the MC.

You're looking for about .025". Too much and your pedal will be low and too little will hold your brakes on.

 

I replaced the caliper and tried to bleed it. No fluid would come out. So I checked the rubber line first thinking it was blocked. I could blow through it easily, no restrictions. I then moved to the line that runs under the cross member. That was clear also. So I then moved to the abs unit and blew through the line heading to the front right caliper and that was clean as well. The front left brake works fine. I have the car on a lift so I inspected the lines front to back and there is no dents or imperfections in any of the lines. Also he original caliper is free not seized.

 

Starting to sound like you have the rare case of a block inside the unit in the right front branch.

Its a bugger of a job but you may have to try releasing the front line going into the ABS and checking it for flow. If it flows then you'll need to release the right front outlet and check for flow.

If there's no flow then you'll probably find it easiest to remove the whole unit from the car and try to clear it on the bench by blowing back in the reverse direction. If you can't clear it then I'm wrong and you will need a good used modulator unit cos you can't service them or buy em new anymore.

I've never had this problem so if there's an easier way or alternate suggestions hopefully someone else will pipe up.

Edit:
You probably either have to plumb the unit up to your MC with the input attached to the right front outlet to get enough pressure to clear it or take it into a hydraulic repair shop. A bit of debris jammed in place with 1500 - 2000 lbs of pressure could take quite a bit of reverse pressure to blow out.

Its a good reason not to push the caliper pistons back too far when changing disc pads. Any rubbish in the line up to the caliper can get pushed back into the ABS which can have some narrow passageways, but not saying that's whats happened in your case. It could have come from the MC end as well.

 

Trash in the system from the MC or simply floating from calipers will eventually get to the ABS unit. I was having intermittent ABS issues until I completely flushed my brakes and cleaned all the crud out of the system...it instantly got better and has been fine ever since.

I would d/c the lines out of the abs BLOCK, then pump the brake pedal. You need the usual pressure in the system to see IF there is a blocked port on the block, or IF fluid DOES flow from the fitting on the ABS block that goes directly to the problem caliper, that tells you the problem is in the lines.
The ABS is full of tiny orifices that will not tolerate a grain of sand. The MC and calipers are full of rubber and metal that all wear and circulate in the system...Add heat and you have blockages. Always change the fluids at the 1st sign of a change in color or ANY particulates suspended in the fluid...

IF you find the ABS port IS plugged, you can TRY to blow it with an air gun, or if you feel lucky, use a dental pick to probe in the hole. That MAY be enough to dislodge the blockage. After each "probing or blowing" in the port try the pedal again to see if fluid will then flow. If none of that helps, you can make fittings to fit that port, engineer fittings to fit a NEW clean grease gun, and make a length of tubing to fill with fluid and use the grease gun to generate the pressure. DON'T pump it enough to push grease in, but if you have some fluid between the gun and the block the grease will force the fluid IN the block. A grease gun can apply a few TONS of fluid pressure...
There is always the possibility of damaging some seals or other internal parts of the block/modulator...but at this point you have absolutely nothing to loose.. Either way it does not work and it MUST before you can drive. You might want to locate the replacement before doing any of these attempts at clearing a plugged block...just in case.

if all that fails, start shopping FleaBay for another ABS unit...there are bunches out there. Also look at Porsche and Early Mercedes parts. They used the same ABS system. May be some stuff available there.

 

Ok, After the line was disconnect from the ABS unit I used a compressor to blow through the line and the fluid came out the other end. when it was disconnected the fluid did not come out of the unit or the line on its own.

Now went back to the car and pushed the brake pedal with the car running and fluid came out of the unit. So the ABS is either clear now or was fine. Thanks for all of your help. Will replace all the fluid for the entire system and bleed everything. I will keep giving an update.

 

"So the ABS is either clear now or was fine."

Good to hear you're making some positive progress but it would be a worry if it was clear all the time. You'd be hoping to have seen a lump of rubbish come out

Looking forward to hearing how it turns out.

 

Ok.....so the line was cleared and everything was connected. Went to bleed the front right caliper. I got fluid to it fine. Now the brakes won't release. All the brakes are locked and won't release. ABS unit or Master cylinder? any suggestions will be appreciated.

 

in theroy these systems should not hold pressure but I have seen them to it a fair amount..

 

Sounds like your MC is holding residual pressure as having blockages in both front and rear circuits at the same time is very unlikely.

First test is to loosen the two nuts that hold the MC to the power booster (PB). Loosen them enough to get a gap of about 1/8" or more. If your brakes release then either the PB rod adjuster is adjusted without clearance and is holding pressure on the MC piston or, if you have had the engine running, a faulty power booster may still be holding pressure on the MC piston. Try removing the vacuum line from the booster to release all vacuum and see if brakes release? Other possibility is a binding brake pedal?

If neither of these are at fault then tighten the MC back up and crack the brake lines at the MC (use plenty of rags) and see if the brakes release? If they do then your MC is at fault.

Unless you feel competent in overhauling your MC then easiest fix would be a new MC at this point.

Others may have some suggestions ...?

 

Cruzin, Thanks for your response I will try this and let you know. Your suggestions are appreciated!

 

Cruzin, I have preformed the three above tasks and with no success. The brakes are still not releasing. Should I pull the ABS unit and try and clear it and all the lines? Or what would you suggest?

 

Are you saying that all 4 wheels remain locked when both brake lines are released at the MC?

How do know the brakes are locked? Whats your test?

Just asking because if you have the car jacked with the wheels off the hubs/discs will appear stiff to turn just because of the pads dragging on the disc's. Some drag is normal but you should still be able to turn the discs easily by putting a long screwdriver through the studs. Just want to make sure you're not expecting the hubs to spin freely?

If your brakes are locked solid with the MC lines released then you'll need to release the lines into the ABS from the MC. If the brakes release then the lines from the MC to ABS are blocked.

If that does not do it then you're back to releasing the three lines coming out of the ABS (one to each front and one to the rear). If that releases them then your blockage is still in the ABS. If that fails to release them then try opening the caliper bleeders to rule out the calipers being seized on the guide pins. If the individual brakes release then your blockage is in the ABS to caliper lines and you'll need to take off the flexible hoses and check them and the steel lines for blockages.

By doing this systematically you should be able to pinpoint where the blockages are. But as said above having either both feed lines into the ABS or all three outlet lines blocked all at the same time is quite bazaar?

Just make sure they are actually locked and not just dragging a bit. A good run around the block using them may free them up. Of course if you can't back out of the garage then that is not an option.

Your answer has to be in this process somewhere?

 

Ironically enough I have done what you have listed. As soon as the line was released between the caliper and the ABS the fliud rushed out, not from the ABS but from the caliper. The rotor could then be spun. It was the same for the front left side. The back brakes were not locked as bad so the pins were cleaned and greased.

So for now the ABS unit will be bypassed to get the car on the road. The entire systenm will be bled and fluid replaced. Thank you very much for your time and wisdom. You have been a great help with this project.

 

You've certainly had your share of problems and glad to have been able to help a little.

 

Just wanted to update. The ABS unit has been bypassed and the brakes are working properly. I tried several time to clear the blockages in the ABS unit. Several small particles came out. However the port for the right side caliper was still blocked. Thanks again for your assistance!