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1970 240z brakes will not bleed

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kirkE

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I have a 70 240z and the brakes have always been soft from when I originally restored the car. I finally decided to upgrade and fix the issue. I just put on the new Toyota front calipers but I still have the drums on the rear. I have pulled a gallon of brake fluid through the lines but I still get air bubbles out of the rear wheel cyclinders and the pedal remains as soft as ever. Here is what I have replaced

1.) New 1" MC from Datsun Parts - figured the 30% extra fluid would fix it. Nope

2.) New vacuum booster from O'riely's - checked the old vacuum boost for reaction disk- fixed that - it didn't help and so I bought a new one - no help

3.) Took off and cleaned the proportioning valve

4.) Hooked up my air compressor to the brake lines trying for force fluid out (somewhere - anywhere) and find what I assume is an air leak - no help

5.) Found a leaking rear wheel cyclinder, fixed that, no leak now but still get air out of the rear

6.) I have bleed the MC multiple times, gravity feed, vacuum pulled, and even done the old two man method and I can't get these things to firm up?

Any advice??

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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.

I don't know why the rears would still have air in them, unless you have them on the wrong side? Don't know if that is even possible.

Good luck,

Marty

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Marty

Thanks for the advice. I will give it a shot this weekend. I am at wits end on this? I think I will pull the rear wheel cylinders and just clean them up as well, although I can't find any leaks and they are only a couple years old, That said the brakes have never been firm? I also tried to backfeed the fluid and push it from the rear wheel forward, but the system must have a check valve or something because I could not get it to take fluid?

Thanks again for the suggestion

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Have you started with the master cylinder itself first?

Make sure the calipers have the bleed nipple on the top and not the bottom. Most common mistake right there

Secondly, you should have never thrown those calipers on there. The stock brakes with nice pads, rebuild calipers and some brembo blanks will bite real hard. The bigger calipers mean u need to upgrade your master cylinder to a 280zx or something that is bigger.

Make sure ur nipples on the calipers are facing up and not down...if they are down...switch the inner part of the caliper right to left, left to right.

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Here's my experience with "brakes that won't bleed."

Start completely over. Assume NOTHING that you fixed, replaced, tightened is OK, because, OBVIOUSLY, at least one of them isn't. Don't say to yourself, "Well I already tightened that fitting or replaced that slave cylinder!" Sometimes even a new part is no good.

It helps if you can isolate your problem to front or rear, consider disconnecting and PLUGGING the line to the rear to see if you can figure out which end has the problem.

If you want MY GUESS, you probably have a line on the proportioning valve or MC that isn't sufficiently tightened down. Hopefully you have a set of brake line (flange) wrenches and can really snug the crap out of them right to where they're about to strip off, as that's often necessary to get a good seal.

I've torn my hair out over brakes that won't bleed, but a slow, start completely over, methodical approach is the only one I've found that works.

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Marty

the brakes are better....but...still not as firm as I would have expected. After isolating lines, I finally took it to a mechanic who said that he could not find any air in the lines? He said he adjusted the rear drums and while the brakes work better, I still get about half a pedal of travel before I get any brakes? I still think most brakiing is at the end of the pedal travel. I think I am going to increase the length of the vacuum boost rod to see if there is something with the engagement at the MC. Maybe I am expecting too much "modern" performance out of these brakes. They do stop well but I would just expect that at about 25% pedal travel I would get some kind of braking action???

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Are you using the stock brake hoses? If yes, replace them with stainless steel braided hoses. They will also help get a firmer pedal. Make sure the brake pedal itself is adjusted to the correct height. Is the reaction disc in the booster in place?

Just some additional thoughts I had. You can get these old brakes to perform very well. The pedal on my track car was quite firm. It had Toyota 4x4 calipers, with 300 Zx vented rotors, braided lines and Axxis Ultimate pads. That thing would throw you through the windshield if you weren't harnessed in. and that was with the stock rear shoes.

Keep working on it, you'll get there.

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Marty has given great advice. make sure the rubber hoses have been replaced. I have also done the old racer trick of reverse bleeding to find trapped air- it works!

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