View Full Version : Truck brakes

08-07-2015, 10:26 AM
ABS module?

On my '96 Dodge 1500 4x4 5.9L (360) (I know - Dodge - don't give me too much grief).

This one followed me home. When I bought the truck about a year and a half ago it had been sitting for over a year. On the test drive the truck still ran great but I could tell the left front caliper was sticking so...

Replaced the Master Cyl, Booster, hard lines (all of them - full length of the truck) flex lines on the front and rear, calipers, pads, rear wheel cyls and shoes - basically a whole new system. Didn't replace the rotors or drums they looked good and didn't vibrate so figured they were not warped or out of round. Bench bled the master cyl first then bled the system thoroughly with new fluid using the method described in the service manual for bleeding ABS systems. Clear (no air) fluid out of all the bleeders on all 4 corners. And I bled them a few extra times just to be sure. You unplug the ABS module (or disconnect the battery) and let the fluid flow through the ABS module. Only needs to be cycled electrically if the ABS fluid module was replaced (according to the manual).

Worked great - never had any trouble for over a year and a half. Didn't like the quality of the re-manufactured booster but it worked so didn't worry about it.

Last month I was leaving a car show and when I stepped on the brakes it was like not having any boost. It's had a vacuum leak since I bought it so....

Replaced the intake manifold gaskets (5). Sure enough the gasket on the bottom of the manifold (2 piece manifold) was bad. Figured this was why it wasn't building vac pressure for the brake booster.

Nope - same problem. Only happens about every 10 or 12 brakes but sometimes 2-3 times in a row. Seems like no boost. ABS light does NOT come on. Really have to stand on the pedal to stop the truck so.....

Replaced the booster and check valve that I didn't trust.

Nope same problem.

Going to replace the Master Cyl with a better quality - just used the cheap-o from Advance but even the cheap-o's should work for more than a year. The manual only lists a way to verify the ABS module is getting voltage and not dead - basically "how to diagnose an ABS light on the dash". My ABS light is not on but cycles on when starting the truck so I know it's not burnt out and the ABS worked this winter on the snow and ice.

Any ideas? Anyone fooled much with ABS systems? This one is a Kelsey Hayes system.

Probably just a bad master cyl but thought I would see what you guys have seen.


08-08-2015, 03:38 AM
...Any ideas? Anyone fooled much with ABS systems? This one is a Kelsey Hayes system...The only suggestion I can offer is to check your electrical connectors closely. There might be corrosion that is hit and miss under your hood.

The connectors used today are not nearly as robust as the bullet connectors Ford used back in the day. I have more faith in your Kelsey ABS than the Teves systems that GM used on Buick Reatta.

Normally, we don't associate 'electrical' with brake systems but the ABS brakes certainly are controlled by electrics. I hope this is helps, Eric. - Dave

08-08-2015, 12:29 PM
You're right Dave - I need to clean the connectors and put them back together with some anti corrosion gook. I "looked" at them but didn't actually clean them. I really hate problems like this - especially with the brakes. This one might have to go to the dealer and see if they have any diag tools/systems to check the ABS system.
Probably wind up spending more on this problem than I paid for the truck.:mad:


08-08-2015, 01:16 PM
Think of it this way Eric, it's already 'broken' and you might get lucky and find a broken or corroded wire at the connection. If that is the case, it's easy to shunt your own connectors around their plug. There's no harm in looking, you might find the problem and you're apt to save some money. - Dave

09-06-2015, 02:26 PM
No real progress on the truck (I'll get to that in a minute) but did find something interesting about our GM proportioning valves most are using on their disc conversions.

On the later MGB's I've worked on for many years there is a valve in the brake system located on the fenderwell referred to in the manual as a "shuttle valve".
Looking at the internal workings the feed from the master cyl for the front comes in and is split. If a front caliper fails the shuttle valve cuts off the failed caliper allowing the remaining good one to continue working and it also lights the brake warning lamp.


I had always A$$umed that the GM combination valves we use on our conversions worked the same way allowing only one caliper failure. BUT NO!!!

If I understand it correctly looking at the cut-away - one front caliper failure on my Tbird would result in the combination valve moving the piston to light a brake failure light ONLY. It does not keep the opposing caliper operable similar to the shuttle valve. The rear would still work because they are separate chambers in the master cly.

Something to keep in mind - no front brakes on these cars would make a huge difference in stopping distance but would definitely be better than a system failure with the original single line system.

(image snagged from the web)
http://image.superchevy.com/f/16643015+w750+st0/sucp_0402_05_z+1968_chevelle_brake_booster+combina tion_valve_cutaway.jpg


and on the Dodge Kelsey Hayes AWAL (all wheel anti lock) system. Apparently there are no rebuild kits, no rebuilt units, no new units, and no one rebuilds them. About the only choice is to buy a used one (around $350) and hope it is better than what you have - crazy. Waiting to hear from the dealer Tues.


09-06-2015, 03:54 PM
Eric, better re-examine the combination proportioning valve again...
Pressure comes in from each side of the M/C to the top of the valve at equal pressure because the piston in the M/C is the same diameter on both sides.

The output sides (both of them) separate the front from the rear systems BUT the two front calipers are in parallel and the two rear wheel cylinders are also in parallel. That means the two front calipers are piped together and the two wheel cylinders are piped together. Those are the two circuits.

Since both input sides come in at equal pressure, if EITHER output system ruptures it will be at zero pressure.The side with pressure will push the center spool toward the ruptured side. This shuts off fluid from running out the ruptured side and the spool completes electrical contact for the brake warning light.

The idea is to make the playing field even for the front disk system with the rear drum system. Disk brakes require much more boost than drum brakes so the valve tames the rear system down while delivering full pressure to the calipers.

The shuttle valve you posted is for a drum/drum or disk/disk system, showing no differential capability. - Dave

09-08-2015, 10:01 AM
Looking at the cut-away below on the proportioning valve I see the inlet for the front system to the left of the pressure failure switch. On the "spool" and to the right of the front inlet hole is a seal to prevent fluid from reaching the switch and the other side of the system. To the left of the inlet there is a small seal but guess that is just for the metering cup to rest on because you need the fluid to flow toward the metering valve and out the front outlets. It actually looks like that seal would move into the large metering chamber if the valve moved left.

I agree that if either front caliper or line fails the pressure will fall to zero and move the spool to the left. To me it looks like the valve spool would only move to the left about 1/8 - 1/4 inch. Probably missing something but I don't see any way that would shut off either of the front outlets , stop the front fluid from leaking out or give you partial front braking as with the shuttle valve on the MGB system. I agree the rear brakes would still work but that is what I mentioned earlier - it would be better than nothing to have rear brakes but stopping the Squarebird with the rear drums might take a mile or ten. :D

And actually the shuttle valve "is" an MGB shuttle valve not just a generic pic. MGB's are front disc rear drum. I believe if any metering/proportioning is needed it is accomplished in the Master Cyl. That shuttle valve is just a safety valve to give you at least one front brake if the other fails. MG added it on the later cars - about '72 IIRC.

The early MG's ('56-'59ish) cars were 4 wheel drum and the only change to add disc brakes in late '59 was to add an extension on the existing master cyl to allow for fluid loss due to brake pad wear (well - other than add rotors, calipers etc). The master cyl did not change.

Maybe the cars are so small and light the system doesn't need any proportioning. It's still a single line system on those early cars similar to the original TBird with no safety valve to shut off a failed caliper or wheel cyl. Even worse the clutch and brake master cyl share the reservoir so if your clutch slave leaks and your fluid gets low - not only would you not have a clutch but your brakes could be gone too. :eek:

Tell me what I'm missing on the GM proportioning valve......


09-08-2015, 11:53 AM
We need to agree that both top ports are populated by the M/C output lines. Both of them. One controls the front system and the other controls the rear system. So the valve is divided between front and rear PRESSURES with a mechanical spool in between that should never move.

The front brake output lines are tied together. The rear line has only one port so they are tied together.

The GM valve shows two outputs for the front system and one for the rear system. If you blow into either of the front output ports, air will freely come out the other front port. They are simply tied together, hydraulically.

It wouldn't make much sense to proportion between two front calipers. So, the GM spool only moves if there is a problem, and then it shifts toward the side that is low in pressure, between the front system and the rear system.

There are several ways to 'balance' a brake system. Sometimes they make rear brake surfaces much smaller. Sometimes they make the wheel cylinder pistons smaller so they don't deliver as much force against the pad or shoes. This can be done in a direct ratio from front to rear systems, in ratios of surface areas and/or piston sizes.

Squarebirds use the same shoes in front as the rear so surface area is the same.

The front cylinders are 1-1/32" or 3.239-sq. in. At 1,000-psi = 3,239 pounds of force pushes against the shoes on each front wheel.
The rear cylinders are 29/32" or 2.847-sq. in. At 1,000-psi - 2,800 pounds of force pushes against the shoes on each rear wheel.

By all accounts, the shoe brake system on Squarebirds is sorely inadequate. The car needs far more room to stop than a comparable disk brake car.

Regarding just the rear wheels braking in an emergency situation, let me remind you that your emergency brake does the same thing. Rear wheels are NOT the main stopping force, but they help and you should be able to lock them up with your pedal. - Dave

09-08-2015, 12:39 PM
Yep - agreed.

Left of the switch for the front and right of the switch for rear.

One output on the rear (far right side of the valve) - ah ha - just figured something out here - keep reading.

It appears you "could" have only one output for the front calipers as well on the GM valve since both are tied to the same metering chamber then split to each side of the front.

This (below) is what I didn't agree with however I think we are saying the same thing. When I say "side" I'm meaning left and right caliper like with the MGB shuttle valve and when you say side I think I've figured out you mean left and right side of the valve which correlates to front and rear of the car. Only took a big hammer to get it though my old hard head - thanks!

"This shuts off fluid from running out the ruptured side and the spool completes electrical contact for the brake warning light.


Emergency brake - funny - it's gonna be more than an emergency if you are yanking on that handle on the Tbird to try and get the car to stop. Everybody in the car will need to make a trip to the laundry mat - pew - :D


Oh - and - got a call from the dealer on the Dodge KH - all wheel anti lock system. "Uh - we're still checking and will have to get back to you. We can't locate anything new or rebuilt" (which is what I told them). :rolleyes: