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97 4-runner, 89 pick
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13 Posts
Discussion Starter #1
89 4X4 Pickup

Both front brakes are building up brake pressure. Enough drag to slow the truck down. The pedal is at the top and hardly requires any pressure or downward movement to stop the truck. I can drive it but, I can feel the drag and smell the
heat from them. Some times, if I jam the pedal hard it will free-up just a little.
It seems to loosen over night but builds pressure again after driving a little.
I had replaced both calipers, rotors and pads in the fall. The problem started a few weeks later. I only drive the truck maybe 10 miles a week.

My mechanic said he released the pressure on the calipers by cracking the brake line at the MC. This tells me that it's not a collapsed hose or frozen calipers. He feels that it's the brake booster but everything I've read about failed boosters describes a pedal that is hard to push down in order to engage the brakes. Thats not what I experience.

The truck was new to me when I replaced the brake components. So I have no history on it. The master cylinder looks clean and might had been replaced not so long before I bought it.

Could the adjustment screw in between the MC and booster move on it's own and cause this problem?

Could the MC cause this?

Can a booster have these symptoms?

Any help appreciated?

Thank you
 

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92 pickup 4wd 4cyl
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21 Posts
After your mechanic "released" the pressure at the MC did the brakes work fine after that?

Cracking the lines at the MC is not necessarilly a sure sign of anything.....except for now you may have air in your lines.

When you changed your brakes did you hang your calipers from a coat hanger or anything or did you let them hang?

Typically when you have a bad MC you have less pedal and less brakes and would possibly need to be pumped to stop. A faulty booster can cause the pedal to be hard and require more pressure to stop the truck.

Based on what you said above, I believe that the culprit could be calipers, but more likely hoses. It could also be debris floating around in your lines. First I would re-bleed the system with some fresh new fluid and see what happens, then re-evaluate the issue :thumbup:
 

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89 p'up
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849 Posts
IIRC this is a sign of brake lines (flexible) failing. Apparently the inner rubber line starts to separate from the outer rubber, and pressure builds up. If this is the case, a new set of rubber lines should fix it, then bleed everything again per the manual.

Good luck :)
 

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97 4-runner, 89 pick
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13 Posts
Discussion Starter #4
I was hoping it was just a simple master cylinder replacement or a hose. My Mechanic claims that there was a substantial release of pressurized fluid when he cracked the fitting. He claims he then removed the vacuum hose and drove the truck around with out a problem. My mechanic feels confident that it's booster.
His thinking is that some valve in the booster isn't functioning and is directing vacuum to the wrong side of the bladder or some crap like that. I asked him to separate the MC and booster and check the adjustment screw between them. It seems to me that if the screw was working it's way loose and not allowing the MC shuttle to return to it's proper resting position, that might cause this type of symptom. I'm not buying the booster is bad...or at least not from him for $360 and three hours of labor. It's been a brutal winter here in the NE and I don't have a dry place to work on it. I might have to park it until the weather permits me to get under it.
Lets say the hose is collapsed, why would there be all this back pressure in the line? Wouldn't the MC allow the fluid to return to the res when the pedal is not depressed? I would expect all the pressure to be contained between the caliper and the hose.
 

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89 p'up
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I was hoping it was just a simple master cylinder replacement or a hose. My Mechanic claims that there was a substantial release of pressurized fluid when he cracked the fitting. He claims he then removed the vacuum hose and drove the truck around with out a problem. My mechanic feels confident that it's booster.
His thinking is that some valve in the booster isn't functioning and is directing vacuum to the wrong side of the bladder or some crap like that. I asked him to separate the MC and booster and check the adjustment screw between them. It seems to me that if the screw was working it's way loose and not allowing the MC shuttle to return to it's proper resting position, that might cause this type of symptom. I'm not buying the booster is bad...or at least not from him for $360 and three hours of labor. It's been a brutal winter here in the NE and I don't have a dry place to work on it. I might have to park it until the weather permits me to get under it.
Lets say the hose is collapsed, why would there be all this back pressure in the line? Wouldn't the MC allow the fluid to return to the res when the pedal is not depressed? I would expect all the pressure to be contained between the caliper and the hose.
I really don't know man. Replacing the booster should be pretty easy for you though, if he's right. Don't forget to bench bleed the MC if you do it.
 

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Soylent Green sales
rock crawler
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12,801 Posts
To see if it's the booster, why not just disconnect and plug the vac line going to it. Sure, the brake pedal will be affected, and you should "release" the pressure or what ever first so the brakes work properly before the test.

Second, what I've seen w/brake hoses going bad is they 'balloon' out at the weak spot when pressure is applied, reducing brake action, not restricting fluid flow.

Third, it could be the MC is bad and not letting enough fluid return to the resevoir when the pedal is released.
 

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97 4-runner, 89 pick
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13 Posts
Discussion Starter #7
UPDATE:

Replacing the Master did fix the problem of the system building up pressure causing the front brakes to slowly lock up.

SIDE NOTE:

I went through two VIP rebuilt MC's that were bad. Both master cylinders would not bleed on the front brake fitting. The rears would bleed fine. The fronts would not build pressure. If you bench bleed the master, both outlets should weep fluid without depressing. Bench bleeding real slow without pushing the shuttle all the way in may be key to not screwing up the seals inside.

Again, If you mount the MC with fluid, make sure both outlets weep fluid before connecting the lines. No fluid weeping, return it.

Thank you all and I hope this thread serves to keep other Toyota owners from getting screwed by piss poor mechanics
 

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91 4x4 extracab
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608 Posts
it sounds like that maybe the old master cylinder wasn't returning to neutral that maybe the adjustment rod might have needed adjusting just an after thought.like most parts each part depending on who it was made by are slightly different.never assume they are.it is hard getting a vehicle and not knowing what was done to it before you got it.i am am glad you fugued it all out though,sometimes it is just hit and miss thats just good troubleshooting.to test a booster you with the engine shut off pump down the brake pedal until all the vacuum is released,then holding the pedal down start the engine if it is working properly you should feel the pedal slowly do down and get firm.
 
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