Rear disc brake issue

[IdahoKidd]

Helping a friend on his 75 280. Just put on toyota 4 piston calipers on the front and ZX calipers on the rear. New steel braided lines on all four corners and a new 15/16 master cylinder. All the calipers are new/reman.

The pedal seems to go halfway down before any "feel" and more to the point, there is very little pressure to the left rear caliper. If we pump up a half dozen times or more, there is movement in the caliper piston and it does hold until the pedal is release. The feel in the pedal remains the same. I have been reading posts on this conversion for some time as the exact system is going to go on my 72, but I don't recall anything on this particular issue. I suspect a defective part, but am not sure. Seems I recall something about adjusting the pushrod from the booster to the master to get a full throw, but haven't figured out how to get that adjustment. Adjusting the rod under the dash doesn't seem to do it.

The system has been thoroughly and repeatedly bled. I will keep reading posts but was hoping someone might catch this and have a thought/solution.

Thanks!

Leonard

[GTTED]

Where did you get the calipers from? I ran into the same problem. I got my calipers from ebay. They looked like they were rebuilt and ready for install, wrong! After two new master cylinders and bleeding and re bleeding the brake system I figured out the the calipers were junkers that were just cleaned up nicely. The internal seals were shot. They didn't leak on the outside, which is what really threw me off for a while. My next problem was figuring out what they were out of. Thank god for the internet! I figured out that they were out of a late 80's 240 sx. Once I had that info it was easy. Bought the rebuild kit and case closed.

[IdahoKidd]

He bought them from Schucks/O'rielly's. No leaks on these either. Sort of narrowed it down to an internally collapsed braided steel line or a defective caliper.

You know the story, a friend in need is a pain in the a!! Will spend some more time there this evening trying to sort it all out.

[Zed Head]

He bought them from Schucks/O'rielly's. No leaks on these either. Sort of narrowed it down to an internally collapsed braided steel line or a defective caliper.

You know the story, a friend in need is a pain in the a!! Will spend some more time there this evening trying to sort it all out.

Bleeder screws at top of all four calipers? Easily overlooked, it just happened to a guy over on Hybridz and he wrote a whole page about it. I have also read that the Toyota calipers sometimes need a little extra work to get the bleed screws oriented correctly, but I can't remember if it was Toyota on the front or back. Worth double-checking. If they're not at the very top of the internal cavity, there will be an air pocket.

[IdahoKidd]

Checked that. The rear calipers are the ZX ones and visually it appears the bleeder is in the middle of the piston cup, but they are on correctly so one would have to assume it should bleed correctly. Have thought about taking them off the mounts, putting something between the piston and plate (to simulate the rotor) and bleed them out flat. Checked source, they are from NAPA, not that it matters much I suppose. They can't get another at this time. At the moment it takes more time to fool with than I have. Still trying to find the post I read about adjusting the pushrod in the booster, but I may have morphed the thoughts and it doesn't work that way at all. The pedal is just too soft until mid throw.

[Zed Head]

Doesn't the brake line enter in the middle of the piston cylinder? Is it possible to swap bleed valve for brake line?

[IdahoKidd]

I didn't put it together, but you might just be on to something! I'll have to drive across town and have a look. I do recall thinking to myself that was an odd place for the bleeder. Going to head out right now in fact. I certainly hope you are right! and may just be. I'll let you know.

[EverRude]

Here's a scan from FSM I got from another thread. I needed to do this as well, when I replaced my master.

Warning though. Don't pull on the rod to get ahold of it. You pull it out of the socket and cause the rubber reaction disk to fall out which will cuase the same pedal problem but much harder to fix. Have you friend press on pedal to get the rod out where you can get ahold of it for adjustment. Good luck.

Edited April 12, 2011 by EverRude

[IdahoKidd]

Zed Head, I roared across town certain your post was the problem. Talk about a let down. It was installed correctly.

EverRude, I do know for a fact that the push rod was pulled from the master with the intent to get it out (it wouldn't come). Do you have any information or can you tell me where I might find the information on the disc of which you speak? I have no idea if he has an fsm. I have only seen a chilton book at his place. I'll see if I can find anything in my cd version. It is an ebay black market sort of thing that is just poor pictures of the pages and by and large worthless.

Thanks guys!

Leonard

[IdahoKidd]

Have spent weeks fooling with rear brakes on this car but still no solution. I've always considered myself a pretty good trouble shooter but am at a loss.

Here is the recap:

1975 280Z. Installed zx calipers in the rear, (new reman's from Orielly's), braided steel lines to the hard lines and a reman 15/16 master from Carquest. New reman 4 piston Toyota calipers on the front with steel braided lines.

The problem:

On initial application, there is no pedal pressure until the pedal is nearly half way down, at which time it becomes very firm. At this point there is enough line pressure in the front lines to firmly lock the front tires (tested at highway speeds as well). There is NOTHING at the rear wheels. Pumping the pedal quickly at least 4 times firms the pedal and will lock the rear tires (at high way speeds as well). Release the pedal pressure and instantly back to front brakes only.

What has been done;

The system has been extensively and repeatedly bled. The front master was replaced. No change. The rear calipers have been replaced. No change. The reaction disc was checked and is in place. The pedal adjustment has been checked, adjusted, manipulated, no change.

It clearly is an issue with the rear system. There is no leaking, no lines bulging (visibly), no other signs of problems. It truly feels like there is air in the line, but there isn't. Am at a total loss and very frustrated. Read up on the NP valve last night after burning up yet another 5 hours of fooling with this thing. It doesn't seem likely it could cause these problems but down to the 'what ifs'.

Does any of this strike a cord with you folks who have converted the rear to disc brakes? Any other thoughts? Trying to get this car to a local show and it would be great if it would stop reliably.

Thanks!

Leonard

[Zed Head]

You might try isolating the calipers from the lines to see if you can find where the problem lies. If you still have the wheel cylinders from the drum brakes, put them back on and clamp the piston in the bore (vise-grips, wire warp, zip-tie, etc.) so you can build pressure. Orient them so that they can be bled. If you can build pressure then the problem is in the calipers, if you can't it's in the lines or MC. It will take a little time, but certainly not another five hours. If you isolate it to the calipers, reconnect one at a time to see if it's one or both of them.

You should be able to do this without removing the calipers, just the brake lines. There might still be air in the lines, you just need to find where it's hiding. Maybe there's a casting flaw in a caliper, holding an air pocket.

I also know that the dust seal on the caliper piston is designed to push the brake pad off of the disc to keep it from rubbing. Maybe you have some other spring-loaded piece pushing the piston back. Can you eye-ball the piston as you let the brake pedal up, after pumping it up to pressure? If it moves back in the bore a lot, it will be pushing fluid back the the MC. It should barely move.

Just a few thoughts to isolate the problem to a distinct component.

[Marty Rogan]

Try this:

If you still have a spongy pedal, try this trick that a racer taught me a long time ago. Bleed the rears properly first. Then on the right front caliper, remove the outside brake pad. Use some padded channel lock pliers and press the piston carefully into the caliper. It will travel farther than with the pad installed. BE very carelful when you push the pedal down, that you don't push the piston out past the seal. Do that a couple times till you get all the bubbles out. Replace the outer pad. Then remove the inner pad and repeat the process. Then move onto the drivers side and repeat.

Every time I would put new calipers on my race car, I could never get a firm pedal until I did this extra step. After doing it once, it was bleeding as usual.

This was written mostly for front calipers, but it should work for rears too.

Good luck!

Marty