Rear Calipers Not Engaging
Thread Starter
Melting Slicks
Joined: Apr 2003
Posts: 3,110
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From: Red Sox Nation! RI
Rear Calipers Not Engaging
--Just added a couple questions at bottom post
I was going to flush the brake system and was having a hell of a time drawing much fluid out with a vacuum pump. Rear end is up on stands, both wheels off and even having someone hold the pedal down, there's hardly any fluid coming out. Needless to say, the calipers are not grabbing the rotors. Started car, put it in drive, the rear brakes won't even hold the rotor still. Calipers are new, as I just assumed they were the reason for the brakes not grabbing.
Keep in mind, I know zero about ABS. I start digging through the manual assuming maybe it's a proportioning valve stuck somewhere? Would a rear wheel solenoid or valve block fluid from reaching the rear calipers?
I notice there's an amber "Anti-Lock" light that supposed to display on the DIC upon startup (in addition to the standard red "brake light"). Am I correct? Since I've never seen "anti-lock" display on the console as long as I can remember owning the car. I do hear the ABS pump cycle when the car is started.
Any suggestions, while I go check out the anti-lock light situation? TIA!
Late 1986, alum. heads, A/T
I was going to flush the brake system and was having a hell of a time drawing much fluid out with a vacuum pump. Rear end is up on stands, both wheels off and even having someone hold the pedal down, there's hardly any fluid coming out. Needless to say, the calipers are not grabbing the rotors. Started car, put it in drive, the rear brakes won't even hold the rotor still. Calipers are new, as I just assumed they were the reason for the brakes not grabbing.
Keep in mind, I know zero about ABS. I start digging through the manual assuming maybe it's a proportioning valve stuck somewhere? Would a rear wheel solenoid or valve block fluid from reaching the rear calipers?
I notice there's an amber "Anti-Lock" light that supposed to display on the DIC upon startup (in addition to the standard red "brake light"). Am I correct? Since I've never seen "anti-lock" display on the console as long as I can remember owning the car. I do hear the ABS pump cycle when the car is started.
Any suggestions, while I go check out the anti-lock light situation? TIA!
Late 1986, alum. heads, A/T
Last edited by C4boy; Aug 29, 2006 at 02:47 PM.
Thread Starter
Melting Slicks
Joined: Apr 2003
Posts: 3,110
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From: Red Sox Nation! RI
Bogus, thanks. I assumed if it was the proportioning valve in the master cylinder that the piston would have triggered the brake warning light. No light. I disconnected the MC brake line going to the rears and put my finger over the MC hole - I can feel pressure pushing out when the pedal is applied.
I replaced the anti-lock bulb in the DIC. It comes on for a second when starting the car then goes off. Any other suggestions?
I replaced the anti-lock bulb in the DIC. It comes on for a second when starting the car then goes off. Any other suggestions?
Team Owner
Joined: Oct 2004
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From: altered state
St. Jude Donor '05
Unless I misread your first post Id guess you have a lot of air in the system and the rears arent getting enough oomph to work.
Otherwise Id say the proprtioning valve. Not often they go bad, though...
Otherwise Id say the proprtioning valve. Not often they go bad, though...
Thread Starter
Melting Slicks
Joined: Apr 2003
Posts: 3,110
Likes: 6
From: Red Sox Nation! RI
nope, no oomph! I just tried bleeding the rear again. If i put a hose on the bleeder and pump up the vacuum pump, I'm lucky if I fill 6-8" of the tubing with fluid. The pump isn't drawing fast like it would if I stuck the end of the hose into a container of fluid. Same results if I push down on the brake pedal.. The pedal is pretty high and hard and almost no fluid comes out. I'm not seeing the reservoir go down either.
It's like the fluid passage is blocked or it's not being pushed through the master cylinder. If this isn't ABS related, I'm guessing it's the master cylinder? There's nothing really left.
It's like the fluid passage is blocked or it's not being pushed through the master cylinder. If this isn't ABS related, I'm guessing it's the master cylinder? There's nothing really left.
Le Mans Master
Joined: Jan 2006
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From: Orlando FL
All good things to check. I know or at least 1 post where there was no pressure to the wheel cylinder and it was a clogged ABS unit. Now in his case it was only one wheel (I think). He disconnected the input line for the rears at the ABS unit and checked for pressure, he had it. The ABS pump was changed and the problem resolved. You might want to do this same check if if you need to. Best of luck. 
Thread Starter
Melting Slicks
Joined: Apr 2003
Posts: 3,110
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From: Red Sox Nation! RI
I'd like to know if anyone else has ever tried this or can verify this: With the rear end up on jack stands and the wheels off, I put the car in drive. I tap the gas and watch the rotors spin up to about 20 mph, then apply the brakes. The calipers don't stop the rotor from turning until hard pressure is applied on the pedal and at the bottom of its stroke.
I just talked to my mechanic and he says this is normal operation for these cars, because the front wheels aren't moving and the ABS system thinks the car is in a skid (speed sensors are detecting rotation on the rears and none in the front). I'm trying to determine if I'm chasing a problem that doesn't exist, for my own peace of mind.
Also, If you've ever flushed your brake fluid - Did you find it harder to draw fluid from your rears than your fronts? I'm using a mighty vac, vacuum pump on the bleeder.
1986, Auto. So far I've torn apart the master cylinder, cleaned it, bench bled, flushed brake lines.
I just talked to my mechanic and he says this is normal operation for these cars, because the front wheels aren't moving and the ABS system thinks the car is in a skid (speed sensors are detecting rotation on the rears and none in the front). I'm trying to determine if I'm chasing a problem that doesn't exist, for my own peace of mind.
Also, If you've ever flushed your brake fluid - Did you find it harder to draw fluid from your rears than your fronts? I'm using a mighty vac, vacuum pump on the bleeder.
1986, Auto. So far I've torn apart the master cylinder, cleaned it, bench bled, flushed brake lines.
Le Mans Master
Joined: Jan 2006
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From: Orlando FL
I’m going to make a wild educated, common sense guess on this since I am not that familiar with the 86.
I would think that if the ABS caused that to happen, the ABS pump would run (which you can hear) and there would be some sort of light combination on the panel saying that a ABS function is happening. If you want to check out that theory, pull the fuse or disconnect it to remove power. This should be on a normally operating system.
I would think that if the ABS caused that to happen, the ABS pump would run (which you can hear) and there would be some sort of light combination on the panel saying that a ABS function is happening. If you want to check out that theory, pull the fuse or disconnect it to remove power. This should be on a normally operating system.
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Thread Starter
Melting Slicks
Joined: Apr 2003
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From: Red Sox Nation! RI
PCColt, did that too. 
I pulled the gauge & brake fuses - same result. The rears still grab slowly. Which is why I'm slightly hesitant to buy my friend's explanation, combined with my lack of ABS knowledge.
Quick, the jack placement person would be me. That's why I traced the lines from the master to the ABS pump to make sure I didn't do just that. No leaks or crushed lines.
Also, I can hear the ABS pump activate if I speed up the wheels then hit the brake. That will usually stop the wheels, although still slowly.
I pulled the gauge & brake fuses - same result. The rears still grab slowly. Which is why I'm slightly hesitant to buy my friend's explanation, combined with my lack of ABS knowledge.Quick, the jack placement person would be me. That's why I traced the lines from the master to the ABS pump to make sure I didn't do just that. No leaks or crushed lines.
Also, I can hear the ABS pump activate if I speed up the wheels then hit the brake. That will usually stop the wheels, although still slowly.
Le Mans Master
Joined: Jan 2006
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From: Orlando FL
I have never done this test myself and have no comparison test results for a much more accurate opinion. I also can’t quantify “still slow” and use that info for a good answer. According to documentation on how ABS should work is if it is disabled, you should just have your regular power brake system as normal braking with no ABS input from activation.
You said when bleeding there's hardly any fluid coming out. This obviously is a restriction with no ABS active or systems on. Try removing the line into the ABS pump unit that would be for the rear brakes, from the master. Check to see if you have good flow there when you press the pedal. If you do, re-connect it and remove a rear wheel line and check for flow again. If there is no flow, the ABS is your problem. If there is flow, then work toward the wheel area.
You said when bleeding there's hardly any fluid coming out. This obviously is a restriction with no ABS active or systems on. Try removing the line into the ABS pump unit that would be for the rear brakes, from the master. Check to see if you have good flow there when you press the pedal. If you do, re-connect it and remove a rear wheel line and check for flow again. If there is no flow, the ABS is your problem. If there is flow, then work toward the wheel area.
Thread Starter
Melting Slicks
Joined: Apr 2003
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From: Red Sox Nation! RI
PC, part of the problem here is my lack of understanding of how the ABS system works. I was trying to find a diagram of the internal workings of the pump to get a better handle on how it operates, but no luck. According to the manual, the pump has 3 valves. One for each front brake line and a 3rd valve that operates both rear brake lines. If I understand correctly, the valves reduce the amount of fluid pressure based on ECM input from the sensors. I've considered disconnecting brake lines at the pump and checking for flow, but I haven't figured out which lines are which.
I'm usually extremely good with technical troubleshooting and leaning towards a proportioning valve problem in the master. But I'd like to completely rule out the ABS and not just thow parts at the car.
As far as quantifying "slow braking": Non ABS, 4 wheel disc cars I've worked on have always stopped the rear rotor within fractions of a second of applying the brakes. Hit the pedal on my vette and the rear calipers are taking more than a second to slow down, and it isn't a grabbing halt. Best I can explain it. I Appreciate the input.
I'm usually extremely good with technical troubleshooting and leaning towards a proportioning valve problem in the master. But I'd like to completely rule out the ABS and not just thow parts at the car.
As far as quantifying "slow braking": Non ABS, 4 wheel disc cars I've worked on have always stopped the rear rotor within fractions of a second of applying the brakes. Hit the pedal on my vette and the rear calipers are taking more than a second to slow down, and it isn't a grabbing halt. Best I can explain it. I Appreciate the input.
Le Mans Master
Joined: Jan 2006
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From: Orlando FL
Sounds like you have a better understanding of the ABS than most. I can’t comment on the design and solenoid configuration of the 86 because I have a 94 and although principal of operation is very similar the ABS pumps are very different.
Trying to find a diagram of the internal workings of the pump is probably impossible as I have tried also. Best info I have is in the Helms books.
On my pump all lines are marked as well as the books backing up what I see. It would be best to look real hard at the pump for info or try to get some documentation rather than go it trial and error.
Question: If the proportioning valve in the master moved wouldn’t it activate the valve sensor (inserted in the side of the master) and then activate a light on the panel in the car? (I assumed you have this sensor). If this happens it should bypass to the front as you indicated. I would think there might be an indication (or not).
Trying to find a diagram of the internal workings of the pump is probably impossible as I have tried also. Best info I have is in the Helms books.
On my pump all lines are marked as well as the books backing up what I see. It would be best to look real hard at the pump for info or try to get some documentation rather than go it trial and error.
Question: If the proportioning valve in the master moved wouldn’t it activate the valve sensor (inserted in the side of the master) and then activate a light on the panel in the car? (I assumed you have this sensor). If this happens it should bypass to the front as you indicated. I would think there might be an indication (or not).
Thread Starter
Melting Slicks
Joined: Apr 2003
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From: Red Sox Nation! RI
Originally Posted by pcolt94
Question: If the proportioning valve in the master moved wouldn’t it activate the valve sensor (inserted in the side of the master) and then activate a light on the panel in the car? (I assumed you have this sensor). If this happens it should bypass to the front as you indicated. I would think there might be an indication (or not).
I believe my buddy may be right in saying this is the way these brakes operate. I just came in from the garage. Put car in drive, get rear wheels up to 25 mph and apply hard pressure to the pedal. I can hear the ABS pump cycling & rotors stop (albeit slower than I'm use to). Do the same test, foot off the brake pedal, and shift into neutral--rear rotors stop.
Think I'm gonna put it back together and test drive it later. Now I'll go do the brakes on my truck
.....^%$#%@# wannabe mechanic chasing problems that don't exist.
Melting Slicks
Joined: Jul 2006
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From: Chesapeake Va
Originally Posted by pcolt94
All good things to check. I know or at least 1 post where there was no pressure to the wheel cylinder and it was a clogged ABS unit. Now in his case it was only one wheel (I think). He disconnected the input line for the rears at the ABS unit and checked for pressure, he had it. The ABS pump was changed and the problem resolved. You might want to do this same check if if you need to. Best of luck. 