Brakes locked up?????? (1 Viewer)

[TIDER96]

First off I have a 73 FJ40. It has disc brakes on the front and drums on the back. Just had new brakes installed-new calipers, pads and rotors for the front and drums and cylinders for the rear. Still have original brake lines and brake booster. After installing everything and getting fluid to it. The brakes completely locked up. So me and my mechanic bleed everything down, first at master cylinder then at each wheel (DR,PR,PF,DF) and still have same problem. Installed new master cylinder and bleed everything down again starting w/the master. After testing brakes, had good front brakes but no rear brakes "at all". So bleed the rear brakes again, now have good drum brakes but the front disc brakes are locking up again. What the crap??? No leaks, and getting good clear brake fluid when we bleed each wheel down. Suggestions???

 

[JackA]

Make sure the pedal push rod is not adjusted too long.

 

[rkymtnflyfisher]

Make sure the pedal push rod is not adjusted too long.

That’s the first thing I would check.

 

[tjhaga]

Did you replace the rubber brake lines? If old they can let fluid thru when pedal is pushed but can collapse when pressure is released, keeping brakes engaged.

 

[LAMBCRUSHER]

push rod is a culprit but the OPs description lends to give credence to @tjhaga s suggestion of collapsed rubber lines... now, a couple thoughts... masters should be bench bled before tying into the field lines, the field should be bled FROM FURTHEST POINT LINEALLY back to the master... if the master was removed or swapped, the pushrod between the booster and master could be too long for the new master; it’ll need to be calibrated- there is a spec for it. Too much clearance and not much action happens, not enough and the brakes will lock. Same if the pedal to booster is too long. Another thing to check is that the front circuit does NOT have a residual pressure valve. If this was outfitted with discs by a previous owner, that may have been overlooked or the front and rear circuits could have been mistakenly swapped... hth

 

[3_puppies]

was it running and driving before this? wondering about the residual valve? is the new master a disc drum master?

 

[TIDER96]

Make sure the pedal push rod is not adjusted too long.

We've adjusted out and in on the push rod multiple times doesn't seem to make any difference. Do you know the spec for the push rod?

 

[TIDER96]

Did you replace the rubber brake lines? If old they can let fluid thru when pedal is pushed but can collapse when pressure is released, keeping brakes engaged.

No the rubber brake lines were not replaced but looks like a good place to start. Because fluid comes out but little or no return which is keeping the brakes engaged. This seems to get worse the more times the brake pedal gets pushed until the brakes finally completely lock. Another tid bit to add is that the feel of the brake pedal when pushed continues to get harder too.

 

[TIDER96]

push rod is a culprit but the OPs description lends to give credence to @tjhaga s suggestion of collapsed rubber lines... now, a couple thoughts... masters should be bench bled before tying into the field lines, the field should be bled FROM FURTHEST POINT LINEALLY back to the master... if the master was removed or swapped, the pushrod between the booster and master could be too long for the new master; it’ll need to be calibrated- there is a spec for it. Too much clearance and not much action happens, not enough and the brakes will lock. Same if the pedal to booster is too long. Another thing to check is that the front circuit does NOT have a residual pressure valve. If this was outfitted with discs by a previous owner, that may have been overlooked or the front and rear circuits could have been mistakenly swapped... hth

The new MC was bench bleed before installing. The brakes lines were bleed from far-near the MC. The push rod has been adjusted in and out several times but doesn't seem to make any difference. DO you have the spec for the push rod? Also there is a valve on each wheel on the front disc brakes.

 

[TIDER96]

was it running and driving before this? wondering about the residual valve? is the new master a disc drum master?

Yes the 40 has been running and driving fine but needed some new brakes. Yes I'm assuming, I installed the same MC the PO had on it. It has 2 reservoirs marked F & R. (F) The front one, the biggest reservoir supplies the disc brakes up front. Correct?

 

[3_puppies]

Yes the 40 has been running and driving fine but needed some new brakes. Yes I'm assuming, I installed the same MC the PO had on it. It has 2 reservoirs marked F & R. (F) The front one, the biggest reservoir supplies the disc brakes up front. Correct?

correct but if you got a 73 master it would be for drums all around, you would need to remove the residual valve for the front circuit

 

[LAMBCRUSHER]

You can’t see the valve without removing the big hub bushing on the side of the cylinder... gunna need to re bleed of course. Now they push rod... I wanna say it needs about 1/8” clearance- spec is in the FSM

 

[TIDER96]

You can’t see the valve without removing the big hub bushing on the side of the cylinder... gunna need to re bleed of course. Now they push rod... I wanna say it needs about 1/8” clearance- spec is in the FSM

Removed the residual valve from the MC - picture below and bleed down the front brake lines again. Jacked up the front end to check brakes and they’re still grabbing after a few presses.

 

[pb4ugo]

When the brakes are locked up, have you tried loosening the master cyldr from the booster to see if the brakes release?

 

[LAMBCRUSHER]

Also, crack a bleeder and see if a bunch of fluid squirts out; that’ll point to the soft rubber, or a crimped hard line

 

[TIDER96]

Yes, when I loosen the hard line going into the MC the brakes loosen up. I have done a check on my old calipers and looks like the pistons are seized. I've decided to replace the calipers and rubber brakes hoses on both sides. While removing one of my rubber hoses I snapped a hard line on the passenger side, so now I'm replacing that too. While I've gotten everything off and the fluid out, I was wondering if I could use an air hose to spray out the hard lines to get rid of any debris. Of course I would detach the line going to the MC first. Just wondering if anyone has done this or if it's ill advised???

 

[TIDER96]

UPDATE... I replaced the rubber hoses, calipers and pads but still had locking brakes. I then shortened the length of the push rod inside the brake booster to nearly all the way in and finally solved the problem. Brakes work flawlessly now. Hallelujah