73 with 10yr old Brake Bleeding Question

[danb]

Hi Guys and Gals,

Quick History:

I have a white 73 that Ive owned for about 13 years. I bought the car right after high school (1996) rebuilt the motor, had the car stolen in Sept '97 right before I entered the US Coast Guard, retrieved the car in July '98, rebuilt the engine (again), and now the only mechanical gremlin I can't figure out is the brakes. The brakes have been a constant frustration for me for the past 10 years!!!!! (OK, there was some down time in there, but still )

I have the brake setup that is used on the 260z and I think on the 280z??? This has the valve with the brake light incorperated in it (directly after the Master cylinder), and the porportioning valve after that located on the fire wall. The m/c has the front brake reservoir nearest the firewall.

I have replaced every brake component except the 2 valves after the m/c, and the rigid brake lines, only because I haven't been able the find replacements for the valves.

So, I bench bled the m/c in the car. Hooked everything up and started bleeding at the wheels starting with driver side rear first (longest run) and finishing with driver side front (shortest run). As I bleed the rear, everything bleeds through fine. Then I go to bleed the front and everything bleeds fine as well, and I have a very firm pedal. Problem is, when I go back the try to bleed the rear again (just double checking it) nothing bleeds out. I even remove the bleeder screws completely and the pedal is rock hard and wont depress at all. I assume the porportioning valve is bad?????

Also, when I drive the car down the street the rear brakes lock up after the pedal travels about half way down. There is no way to ease the brakes on. Its all or nothin on the rear. I dont think the front calipers even engage.

Please!!!! Any help would be awsome!!!

Thanks in advance!

Edited June 2, 2009 by danb

[Travel'n Man]

I recommend to jack the car up off all fours. Take off all your wheels and remove the rear drums. Have a buddy sit in the car and apply the brakes and you look at each of the pistons from each braking system and make sure each of them are working correctly. It sounds like your rear piston is frozen, if it is you will have a challange getting that drum off. Get two small thin screwdrivers and while using the good rear brake as an example get into the inspection hole of the locked brake and push the star wheel lock down to free up the star wheel, while that is down you will need to counterclock turn the star wheel to free up the drum to remove it with a second scrwedriver in the same inspection hole (its a little tricky but it will free up your drum and save you $100 from beating the hell out of it). If this is the case check that brake piston one more time so you can see if it is frozen (when you press on the brakes it will NOT work and I bet you will see a little brake fluid come out of the piston itself).

Sounds like a bad wheel cylinder to me...................remember, there are left ones and right ones, be sure to order the correct size. I bet only one of your rears are locked, once you jack it up and check you will know for sure, be sure to use jack stands!!

Edited June 2, 2009 by Travel'n Man
forgot an item

[danb]

Thanks for the response! Where Im at right now is the car is sitting on jackstands with all 4 wheels off. I replaced both rear wheel cylinders about 2 years ago. The rear drums will slide on and off as they should during a routine brake job.

I asssume your refering to the piston at each rear brake cylinder right? I havent checked them since I installed the new cylinder. Since the car has been sitting for quite some time, could those have gone bad in a couple years time? Also, I can get the rear brakes to bleed ONLY if I do them before the fronts (the way they are suppose to be done). Once I have bleed the fronts, thats when I can no longer get any fluid to the rear.

[motorman7]

I currently have the same problem with my '70. I cleaned and exercised the proportioning valve; honed and lubed the rear brake cylinders (they were froze); and still no luck. The front brakes work great, the rears do not operate. Fluid drains from rear brake reservoir when rear brake lines are open, and even fills system when 'bleeding', but no pressure. I just got a 'major' rebuild kit for the Master Cylinder and will replace that tonite. I am hoping that solves the problem. Not much left to the system. Only the junction that has the brake warning light.

[danb]

Ya, its been a long process getting to this point. I've tried everything that friends and fellow Z enthusiests have recommended, but still no luck! I dismanteled the proportioning valve, cleaned it, reassembled it, and it seems to allow brake fluid to bleed through for the rear brakes perfectly...... as long as I dont bleed the front brakes. Once the fronts are bleed, no fluid moves to the rear brakes. It acts as if the piston inside the proportioning valve is stuck...but only after I bleed the front brakes. To weird!

Good luck with your car! I assume you have the stock early 240 setup (without the front/ rear prop. valve) ?

[motorman7]

Well, that fixed the problem. I installed a "major" rebuild kit from Tokico (Same manufacturer of Master Cylinder) and the rear brakes finally bleed and brake. Of course, its the last item I check. Worked from back to front fixing parts. Anyway, I feel good about the whole system now that I have gone through it. By the way, the rebuild kit was so cheap that I thought it would be cheap chinese garbage. Purchased from Classicgarage.com for $23. The pleasant surprise was the manufacturer-same as original. Best of luck on your Z brakes there.

Rich

[danb]

Right on Rich! Glad to hear ya got it goin! One less Z parked in disassembly. Ya, I'll get it going one of these days. I just have to keep trying diffent things. I havent been able to drive my car for allmost 10yrs now because of these damn brakes. Worst part is my rebuilt motor is just waiting to get broke in, I'm thinking on OR hwy 101!

[danb]

Well.... the latest thing i tried was removing and disassembleing the brake booster. I got it out of another '73 (same year as mine) with good working brakes. When I opened it up there was no reaction disc to be found???? Is this normal? I proceeded to adjust out the pushrod by unthreading the "nut" that inserts into the MC. I adjusted it out approx the the same amount as the thickness of the reaction disc would be.

After everything is hooked back up I drove the car and the brakes did the exact same thing as before I messed with the booster....the rears lock up with very little pressure applied to the pedal.

SO FRUSTRATING!! Im not giving up though.......any ideas or suggestions? I have tried sooooooooooooooo many things and checked every part of the system i can think of.