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Discussion Starter · #1 ·
1996 Voyager 2.4L, short wheelbase, 14" wheels. Today both front brakes overheated after a 4 mile drive. I jacked up all 4 wheels, transmission in "N" and parking brake off. The two rear wheels spun freely, but both fronts could barely be turned. I cracked the two tube nuts at the Master Cylinder, one at a time and tested. Just a little fluid dribbled out, not a spurt like there was pressure held. The wheels were still hard to turn, though perhaps slightly easier. I recall in the past they would spin freely. Currently new front calipers, pads, front hoses, and vacuum booster.

My main question is what might lock up both front calipers hydraulically? The tube from each goes to the "ABS Block". I think in 1996 it distributes the 2 MC pressures in an X pattern, i.e. LF and RR on one circuit and RF and LR on the other circuit. Correct? So, if the left caliper was jammed, the right rear drum should be too, if they were connected (no internal blockage like degraded rubber. I think the ABS Block is like the older "combination valve" in 1970's cars. It serves as both a distribution block, pressure-imbalance switch, and perhaps "front metering valve", plus the ABS pump which pulses the hydraulics. Presumably there are 2 front metering valves since an X-pattern, so are they mechanically coupled so they can fail at the same time?

History: The front left brake had been overheating when my daughter drove it. I had 2 new calipers on the shelf and new hoses, so replaced those and thoroughly bled the system at all 4 wheels, plus new ceramic pads (Autozone lifetime warranty). The old fluid was clear with no rust. I couldn't push the old left caliper piston in, but the rotor had a small enough lip I was able to slide it off. Even off the car with a C-clamp and the bleed screw out, I couldn't push in the piston so it was jammed hard. I assumed that was the problem. It didn't appear to overheat or give any drag in "N" in a test drive 3 miles, nor when my daughter drove it further with the new parts. I later installed a new booster since the old one seemed to require more brake pedal (even w/ new calipers and pads) and sure seems like easier braking with the new booster.

My next test is to try to get the fronts jammed again, then loosen the bleed at each caliper and see if that frees them. If so, I know it is a hydraulic issue. The calipers were cheap from a liquidator, but have no corrosion and looked pristine, and the slides moved easily and I could see a little silicone grease at the bellows so didn't remove the sliders. I also checked clearance on the ends of the new pads (15 mil) against where they slide on the spindle arms and coated the slides w/ silicone brake grease. I'll reverify they still slide fine. I also have the original master cylinder which looks fine, but I replaced it ~6 years ago when solving another problem, so can swap that one in as a test. That problem turned out the be a leaking tube, where a braided section passes thru a hole in the engine cradle which had frayed the stainless wire, allowing the inner tube to grow an "anyerism" and break to leak (location that was hard to find).

Looking for more ideas. Seems if I don't find a problem with the calipers or MC and it is due to a hydraulic pressure lock-up, the main culprit might be the ABS Block, which is likely expensive, and not a fun location to work. All I find is a used one for $80 on ebay.
 

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Discussion Starter · #2 ·
Update.
Good thing is I could at least easily reproduced the problem this morning. Both front calipers were locked up tight after a 1 mile drive. When I opened the bleeders, they both released, so the problem is not in the calipers (stuck pistons, seals, or sliders). Hard to believe that both new brake hoses could be bad, but I removed the right one and bench tested it. It flowed freely both ways under air and I tried multiple tests. Anyway, I bought a new hose for that side since already off. All fingers point to the ABS Hydraulic Block since each front tube comes separately from there. I have read that the valves can get stuck from lack of lubricity, but don't know if there is a common valve which could affect both sides. My guess is there could be from a proportioning or metering system. From a little reading, they don't have those as mechanical functions anymore, but rather are controlled digitally by the control module which attaches to the hydraulic unit, at least in the latest systems. My van has ABS, but not traction control. I'll flush thru new DOT 3 fluid, and try back-flushing it since apparently there are screens inside which can clog. The next step will be replacing the ABS unit.

If so, I will get a used unit since rebuilts cost $350+. In my 1960's Mopars, one can install aftermarket proportioning valves cheaply. I did so, switching to a dual-port Master Cylinder. But in these later X-pattern brake systems it all exists in these expensive brake blocks, and becomes harder to diagnose and repair.
 

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Discussion Starter · #4 ·
I would loosen the fittings at the top end of the flex hose next time. See if that released the caliper.
I didn't quite do that, but I replaced the right caliper hose with another new one (Brake Best from O'Reilly's, seemed cheaper) and both calipers still locked up after a 3 mile drive, and the right one was the tightest (couldn't even budge the wheel by hand).

Since then, I flushed 1 quart of DOT 3 thru all 4 wheels. The old fluid came out clear, so unlikely a corrosion problem. I'm fairly convinced the problem is in the ABS hydraulic unit, but I'll try loosening the MC fittings again since that test left a slight question. Whatever is locking them up is a very tight seal, since they were locked even after sitting a few hours.
 

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Perhaps it is possible to by pass the ABS system. I could certainly do without it, or with the proper scan tool, maybe it can be locked out/open inoperative. What if you released all the pressure so the pistons were back in and you removed the wires from the abs block?
 

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You said after the first time they locked up, you replaced the calipers, hoses, and even the BOOSTER? Sometimes the booster has that little rod that pushes against the master cylinder, and it's the wrong length or adjusted wrong. If it's keeping pressure on the master cylinder, it won't allow the fluid to return into the master cylinder (at least quickly).

Quick test for this is when the brakes are locked up, loosen the nuts holding the master cylinder to the booster. If brakes release, it's the booster. At least this should eliminate everything except the ABS block. Cheapest stuff first!
 

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Discussion Starter · #7 · (Edited)
Thanks for the useful replies.

I might have been dealing with separate problems. The first time, I only noticed the left caliper hot and I was unable to push the piston back in, even off the car. That was the original caliper (bought van new) though maybe on the 3rd set of pads at 230K miles. After replacing calipers, my 19 yr old daughter reported the problem recurring. She isn't detailed, more screaming than informative. She abandoned the van 5 miles away and took Uber home, not even waiting 10 min for me to drive there, so I rode a bike to retrieve it. Brakes didn't lock up on me but I did smell burnt pads. I had bought a new booster since the original didn't seem to give enough boost and seemed to make a little dragging/hissing sound when the pedal was first pressed. Only after installing the new booster did I notice that both fronts were locking up, and could stay locked overnight. The brakes did apply more powerfully and smoothly with the new booster.

The right caliper was found still locked after sitting overnight, though I could turn the wheel by hand with a lot of effort vs not at all the evening before (transmission in "N"). I then repeated cracking both tube nuts at the MC, which freed both calipers. It hadn't seemed to have much effect before, but perhaps I was just working against the drag in the differential. I then suspected Road Ripper's suggestion of the booster rod adjustment. I had done that on the new booster, but didn't know it was so finicky until researching on the internet. I hadn't bothered to account for the 60 mil thickness of a thin metal plate I held the the vernier against, and people say 10 mil is the tolerance. It was hard to measure exactly with the vernier, even with the booster off the car, and even harder on the car (especially w/ battery out but 2.4L air box in). I doubt that the original booster and MC could have become misadjusted, which is why I suspect the earlier lockup was a left caliper issue. I'm going to buy the special measuring tool since I have other cars to fool with.

This pass, I relied mostly on how I adjusted booster tips on my old Mopars, after adjusting as best I could w/ vernier measurements. I removed the rubber sealing ring and pushed the MC against the booster, feeling for when the piston resisted the rod. It felt like as-found the rod was pushing the piston in slightly. I kept screwing the rod tip in until I could seat the MC without feeling any resistance from the piston, then another 2 turns in to be safe. Not surprisingly, that put the rod back about where "as received" (though booster instructions said you should adjust the rod). I then bolted up with the vacuum sealing ring. When pushing the pedal (engine off), it seems I fee; a slight gap before the MC piston resists. Not enough to affect pedal stroke significantly and seems good since the pedal switch then clicks on before the brakes apply, which is useful to flash brake lights at tail-gaters without actually braking. If they spill hot coffee in their lap, that proves educational. TBD if it locks up again. I'll try fairly short drives, with aggressive braking on empty roads and actuating ABS.
Perhaps it is possible to by pass the ABS system. I could certainly do without it, or with the proper scan tool, maybe it can be locked out/open inoperative. What if you released all the pressure so the pistons were back in and you removed the wires from the abs block?
I thought of that, but CA rainy season starts in a week. I doubt I will ever take this minivan back to Lake Tahoe again, since our 2002 T&C has AWD. Long ago, driving the Voyager on the west side of the lake, before I could find a place to pull over and chain up (tailgater trying to rush us), I passed an "icy" warning sign. They weren't kidding. Driving only 25 mph, the van just slid across the road on a turn, with ABS not helping to allow steering. But in snow and rain, it helps a lot, if you keep the pedal jammed to let ABS do its pulsing. Too many drivers become alarmed by the strange "broken" feel when ABS actuates so lift the pedal. Best to experience it a wet parking lot to be familiar.

Two semi-mysteries remain. I could have sworn that one time I tried unscrewing the MC to move it away from the booster and the brakes were still locked. I recall that was with the new booster and both calipers, but I need to take better notes. Another question is if the booster rod is too far out to keep the MC piston from retracting fully to expose the fill ports, how was I able to bleed the brakes? I did this by loosening each bleed, one at a time, with a hand vacuum jar on the bleed, then pumping the pedal. I first remove the bleed and coating the threads with silicone grease so no vacuum leak past the threads. I'll try to remember to report back if success. If an erratic problem, like a chunk of rubber floating in the tubes to cause occasional blockage, hard to know you truly fixed it.
 

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Discussion Starter · #8 ·
Result. Dan was right, the issue was the rod adjustment of the booster. I tried measuring again using a vernier and spacers, but even tougher with the booster already installed. So, I repeated my old-school method of pushing MC against booster (rubber seal washer off). It felt like there was slight contact with the rod before they mated. I screwed in the rod until I couldn't discern that, then another turn in to be safe. No more caliper lock-up since, even after a long drive, so hope it is fixed. There is no significant play when pushing the pedal before the brakes apply, so the rod isn't set too short.

I think I can answer my own question of how the brakes could still be bled if the MC's fill port was never exposed. As I recall of MC tear-downs, the piston seal isn't solid, but a thin flapper which can act as a one-way valve, so it could pull in new fluid from behind it. The area behind the piston might refill from the fill port, and perhaps there is another seal on the rod to the outside. If I take my old one apart I'll ponder it. Perhaps another hint was that I couldn't draw fluid from the bleeder port with a vacuum pump, whereas it should drip out even under gravity (some use "gravity bleed" method). I had to stroke the pedal to bleed fluid. That might indicate that the fill port isn't exposed, like putting a finger on a straw.

Since then, I ordered a $13 tool to set booster rods, which I just received, but a bit late since not revisiting. There are better metal tools for $30, but all seem designed for Corvettes, which don't space the feet wide enough to fit my MC. But, testing the one shown on my spare MC, the legs aren't long enough to reach the base of the MC, given the long extension w/ rubber cover on mine. I would have to use a spacer on the MC and subtract (hate math). At least I can use it on my classic Mopars.
Power Brake Booster Pushrod Pin Adjustment Gauge Tool | eBay
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Result. Dan was right, the issue was the rod adjustment of the booster. I tried measuring again using a vernier and spacers, but even tougher with the booster already installed. So, I repeated my old-school method of pushing MC against booster (rubber seal washer off). It felt like there was slight contact with the rod before they mated. I screwed in the rod until I couldn't discern that, then another turn in to be safe. No more caliper lock-up since, even after a long drive, so hope it is fixed. There is no significant play when pushing the pedal before the brakes apply, so the rod isn't set too short.

I think I can answer my own question of how the brakes could still be bled if the MC's fill port was never exposed. As I recall of MC tear-downs, the piston seal isn't solid, but a thin flapper which can act as a one-way valve, so it could pull in new fluid from behind it. The area behind the piston might refill from the fill port, and perhaps there is another seal on the rod to the outside. If I take my old one apart I'll ponder it. Perhaps another hint was that I couldn't draw fluid from the bleeder port with a vacuum pump, whereas it should drip out even under gravity (some use "gravity bleed" method). I had to stroke the pedal to bleed fluid. That might indicate that the fill port isn't exposed, like putting a finger on a straw.

Since then, I ordered a $13 tool to set booster rods, which I just received, but a bit late since not revisiting. There are better metal tools for $30, but all seem designed for Corvettes, which don't space the feet wide enough to fit my MC. But, testing the one shown on my spare MC, the legs aren't long enough to reach the base of the MC, given the long extension w/ rubber cover on mine. I would have to use a spacer on the MC and subtract (hate math). At least I can use it on my classic Mopars.
Power Brake Booster Pushrod Pin Adjustment Gauge Tool | eBay
View attachment 63881
This thread contains some very interesting info! So the improper length of the booster's pushrod can happen EITHER when you change your Master but retain the old booster, OR when you change the booster but retain the old Master? If that is the case, I wonder why this is not wide known knowledge, like greasing the sliding pins in the calipers, when you change the brake pads, etc, etc?
 

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Hooray for cheap fixes!!

I recently had the driver's front caliper stick/lock up on me. A few weeks ago it started, so I removed the caliper to lube the slider pins, changed a brake pad, pushed in the piston and reset the brake and it was okay for a couple of weeks. Then it happened again. Thought it might be the master cylinder. Nope, it was the caliper again. It's about 9 years old and probably didn't like being used with slim pads and the piston extended (have to run a worn outer pad for my wheel clearance).

I swapped out the rotor and pads for better used parts I have on hand, and swapped out the caliper with an almost new one from the junkyard that I found last year. Yeah, I hoard spare parts but they come in handy at times like this! :ROFLMAO: Got it together and bled, test drove and no more problems! I should probably check the other front brake before that decides to act up on me. :p

I've replaced master cylinders before without messing with the rod, and everything worked as it should. I think the problem is when you replace the booster. This same booster/rod issue pops up on people with the 5th gen vans doing the heavy duty brake swap along with the master cylinder.
 
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