Rear brake caliper won't release....rearend upsizing lead to this? (Sorry long)

notnilc20
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Rear brake caliper won't release....rearend upsizing lead to this? (Sorry long)

Postby notnilc20 » Tue Aug 15, 2017 9:32 pm

Currently trying to finish up my sr5 to gts conversion. I opted for a gm 7.625 rearend out of a s10 blazer. It has disc brakes and the calipers are single piston but are pretty good size.

I am having issues with the passenger side rear caliper hanging up and not letting the wheel spin freely when the brake is released. Thought it was a collapsed hose at first.....replaced it but no help. Both the slider pins are moving just fine. Also when the brake is released, I can't spin the rotor but if I get a screwdriver and put it on the piston and tap on it with a rubber mallett, then it will release and I can spin it then. So I went to the salvage yard and got another caliper.....same thing. I even dissassembled the one caliper to check the piston to see if there is any corrosion on the sleeve but nothing....it looked fine to my untrained eye. Should I get a new one anyway?
????
I'm at a loss here....my other thought is maybe the mastercylinder can't handle the larger rear brakes? But it clamps down just fine.....just won't release. The mastercylinder is new and yes I bled it before install. Maybe I should get an adjustable proportioning valve and adust it to have less pressure to the rear? But remember the driver side releases just fine.

When people do the Ford 8.8 rear end which is an even larger rear end than mine....do they have any brake issues? Do they change mastercylinder to compensate for the larger brakes?

PLEASE HELP! Thanks in advance.

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davew7
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Re: Rear brake caliper won't release....rearend upsizing lead to this? (Sorry long)

Postby davew7 » Wed Aug 16, 2017 4:59 am

You may not have enough free travel in the master cylinder push rod, to release the pressure to the rear axle. [ you have to uncover a bleed port in the M/C.] Do you have a return spring on the brake pedal? You also need 1mm of free play at the master clyinder push rod before it engages. It's also possible that you have a damaged brake line. Dave W

notnilc20
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Re: Rear brake caliper won't release....rearend upsizing lead to this? (Sorry long)

Postby notnilc20 » Wed Aug 16, 2017 8:21 am

davew7 wrote:You may not have enough free travel in the master cylinder push rod, to release the pressure to the rear axle. [ you have to uncover a bleed port in the M/C.] Do you have a return spring on the brake pedal? You also need 1mm of free play at the master clyinder push rod before it engages. It's also possible that you have a damaged brake line. Dave W


Thank you for the response. I will check the master cylinder travel when I get home. From what I understand there should be 0.2mm gap between the brake booster rod and the mastercylinder rod? I would have no idea how to measure this but I will have to go by feel I suppose?

When others have gone to a 8.8 rearend is their mastercylinder adequate enough to handle the larger disc brakes? Also there was a concern about me going from drum brakes to then larger disc brakes if there would be too much of a difference and the master cylinder couldn't handle it.

One more thing....I'm not sure if I have an sr5 or a GTS brake proportioning valve......i just used the one that was already installed but the previous owner didn't know much about what had been swapped or not as he bought it as an existing project as well. would it make a difference if I had a gts or sr5 prop valve? Thanks again.
I'll keep you all posted and any other suggestions are welcomed.

notnilc20
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Re: Rear brake caliper won't release....rearend upsizing lead to this? (Sorry long)

Postby notnilc20 » Wed Aug 16, 2017 4:02 pm

davew7 wrote:You may not have enough free travel in the master cylinder push rod, to release the pressure to the rear axle. [ you have to uncover a bleed port in the M/C.] Do you have a return spring on the brake pedal? You also need 1mm of free play at the master clyinder push rod before it engages. It's also possible that you have a damaged brake line. Dave W


Davew7....I unbolted my mastercylinder from the brake booster today and left it connected to the brake lines to keep the pressure. I was thinking that that would relieve any travel in the MC pushrod and release the caliper. However it did not work. The caliper is still locked up even though the MC is not attached to the booster.

This is so frustrating. I did notice however that I do NOT have a return spring on the brake pedal as you mentioned. Also there was no gasket between the MC and brake booster (not sure if one is required there).

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notnilc20
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Re: Rear brake caliper won't release....rearend upsizing lead to this? (Sorry long)

Postby notnilc20 » Thu Aug 17, 2017 6:57 pm

Ok....I took the spring and rod out of the proportioning valve for the rear portion and still no luck. However, while the car was jacked up....and the brake system open at the prop valve location, I tried to spin the rear wheel and it was still stuck.....shouldn't it have let go pressure when the brake system is opened? So then I opened the bleeder valve at the offending caliper and still it was not letting loose. WTF? So now.....doesn't this isolate the problem causer to the caliper piston? My caliper bracket is good and the slide pins are lubed and move fine. I can't think of anything else. I'll try a new caliper and see what happens I guess.

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Re: Rear brake caliper won't release....rearend upsizing lead to this? (Sorry long)

Postby jinx » Thu Aug 17, 2017 8:30 pm

so you've isolated the problem down to the seal or whatever retracts the piston.
Maybe the parking brake mechanism interferring with that ?
could u first take the caliper apart to see what's up ? ....since u may be using it as a core later anyway

notnilc20
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Re: Rear brake caliper won't release....rearend upsizing lead to this? (Sorry long)

Postby notnilc20 » Thu Aug 17, 2017 8:57 pm

jinx wrote:so you've isolated the problem down to the seal or whatever retracts the piston.
Maybe the parking brake mechanism interferring with that ?
could u first take the caliper apart to see what's up ? ....since u may be using it as a core later anyway


Yeah....I may take it apart just to see what is going on. For my parking brake I don't have cables.....I integrated a hydraulic brake. I even checked this and made sure the cylinder on the hydro brake is fully dissengaged. I'll update here soon.

davew7
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Re: Rear brake caliper won't release....rearend upsizing lead to this? (Sorry long)

Postby davew7 » Fri Aug 18, 2017 5:06 am

If you opened the bleeder valve and the brake is still locked, I would bet that it has to be something with the caliper. If the caliper is really worn the piston can jam inside the cylinder. Also check ALL the surfaces that the brake pads slide on. I assume that when you cracked the bleeder that fluid came out under pressure? Dave W

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Re: Rear brake caliper won't release....rearend upsizing lead to this? (Sorry long)

Postby davew7 » Fri Aug 18, 2017 5:23 am

I would also go back and take another look at the E-brake cable release on the caliper. Make sure that it's not jammed or rusted. Dave W

notnilc20
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Re: Rear brake caliper won't release....rearend upsizing lead to this? (Sorry long)

Postby notnilc20 » Fri Aug 18, 2017 8:16 am

davew7 wrote:If you opened the bleeder valve and the brake is still locked, I would bet that it has to be something with the caliper. If the caliper is really worn the piston can jam inside the cylinder. Also check ALL the surfaces that the brake pads slide on. I assume that when you cracked the bleeder that fluid came out under pressure? Dave W


I cracked the bleeder but it didn't shoot out...just came out like normal. Also, it's a bit strange because when I got a used replacement caliper it hung up as well, so I took the piston out of the used caliper to see what was the deal and everything (to my untrained eye) looked fine. No rust or corrosion build up that i could see. That's why I didn't suspect the caliper at first. I'm still not fully convinced but I'm running out of options here.

notnilc20
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Re: Rear brake caliper won't release....rearend upsizing lead to this? (Sorry long)

Postby notnilc20 » Fri Aug 18, 2017 5:46 pm

UPDATE: I got a new caliper from Autozone (reman unit) and lo and behold it worked! No more sticking! I will take apart the old caliper tomorrow and see what was going on. What a relief to get this situated...I want to thank all you guys for your help and expertise. Esp. Davew7 and Jinx.

davew7
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Re: Rear brake caliper won't release....rearend upsizing lead to this? (Sorry long)

Postby davew7 » Sat Aug 19, 2017 4:43 am

After your first test drive, do not be surprised if you need to get a brake bias valve, [pressure relief] to limit rear braking with the large size disc in the back. Dave W7.

notnilc20
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Re: Rear brake caliper won't release....rearend upsizing lead to this? (Sorry long)

Postby notnilc20 » Sat Aug 19, 2017 6:30 pm

davew7 wrote:After your first test drive, do not be surprised if you need to get a brake bias valve, [pressure relief] to limit rear braking with the large size disc in the back. Dave W7.


something like this perhaps??

http://www.ebay.com/itm/131499815345

You can tie this in after the oem prop valve to the rear side and it is adjustable.

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Re: Rear brake caliper won't release....rearend upsizing lead to this? (Sorry long)

Postby jinx » Sat Aug 19, 2017 9:52 pm

master cyl vs caliper volume, pedal travel, etc., may all affect bias
Before buying any parts, I'd get familiar with the braking needs
will u b upgrading the fronts in the future ?
A simple fr pad compound change can alter the dynamics

notnilc20
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Re: Rear brake caliper won't release....rearend upsizing lead to this? (Sorry long)

Postby notnilc20 » Sun Aug 20, 2017 7:09 am

jinx wrote:master cyl vs caliper volume, pedal travel, etc., may all affect bias
Before buying any parts, I'd get familiar with the braking needs
will u b upgrading the fronts in the future ?
A simple fr pad compound change can alter the dynamics


Probably won't be upgrading the fronts....yeah, i'm gonna roll with just the oem equipment for now and then see what i'll need. I appreciate the advice.

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