Datsun Roadster Forum-311s.org

OK, here goes...
1. Rebuilt the dual brake master on my 69, bled out the system, all good, full fluid flush good pedal. After 6 or 7 stops, loud clunk, pedal dropped, no rear brakes, no leaks, fluid trickles out of rear line bleeder when pedal pushed. Drove with front brakes and low pedal for a few days, no problem.

2 Re Re built master... Found the secondary piston rod was sticking and piston was jammed all the way forward. Fixed, Re assemble, re bleed, drive the car all is well for the 1st 2 miles. The car was slowing down, found the rear brakes locking up, would not release until I loosened adjustment. seemed no big deal... Drove a few miles further, smoke from right front, front brake locked up (My wife had brought the floor jack over for 1st breakdown)
Loosened front bleeder screw, fluid squirted out, wheel unfroze. Drove home 5 miles at night in local traffic without using brakes, nice challenge.

3. Installed spacers I removed and forgot to put back in between master and firewall. Also got smart and knocked the bottom master cylinder mounting bolt out of the brake brace and inserted bolt from front with lockwasher and tightened from the inside, MUCH EASIER.

4. Pump the brakes hard a few times, front brake still locks until bleeder is opened to release pressure.

5. Yelled at dogs and threw tools about the garage, no result, except the Basset Hound now fears the smell of brake fluid.

6. HELP. I'm leaning toward Master cylinder, but if anyone has any ideas feel free to chime in

Thanks

As far as I can tell, it's coming all the way up, slack in brake pedal, but I'll look again. If needed, Does the switch to a 510 master cyl work if you switch the bottom fittings from the old one?

Hey Dave,
I'm having front wheel brake lock on my 69 2000. New master last year and no problem until today. Haven't torn it apart yet. Some of the replies in this thread have been deleted. Did you figure it out? I'm guessing mine is master related but before I pull everything apart - again - I'm hoping there might be another explanation. Thanks in advance.

Have you ruled out sticky caliper pistons? With the front of the car on jack stands, an assistant can push on the brake while you are turning each front wheel.

You may have already addressed this concern, but some of our cars likely haven't had their calipers rebuilt since Reagan was president.

It’s both fronts so I’m assuming it’s the master. I crack the bleeder on the master and they both release. Looking at a rebuild kit as opposed to $8-900 for aftermarket or more for original. Haven’t done a rebuild on one of these before.

Bob,
You can PM dbrick by clicking on "dbrick" under his avatar to see how he dealt with this issue. Once you have the m/c apart you can judge whether it is worth a rebuild, and rebuilding is pretty straightforward if you pay careful attention to the order in which parts are assembled. Good luck!
Jon

If you aren’t 100* set on authenticity, then the 510 master works just as well. Just be sure you get the adapters for the lines. See the tech WIKI

Does it flow out of the MC normal when pressure ized ?
The brake safety swt. The wire goes to.has a floating piston in a cyl. With equal pressure in each line the piston stays put ,but low to no pressure on 1 side it let's a piston slide move towards the low pressure line which turns on the brake indicator lite ,also if it goes far enough like a line blew out, it shuts off any fluid to that line too as not to deplete the fluid level quickly. Bleeding brakes pressing the pedal hard is not recommend for this reason ...to recenter the piston you have to pressure the side it leaked more . There are centering springs that rust or break.
It can be stuck good on older cars . If this was a 1 tank MC it makes sense it would shut off the rears

Have you seen this...it was posted a few weeks back on 311s

http://sincitydatsuns.com/store/p193/Da ... ARKET.html

Usually that part of the cylinder that never meets the piston after years is damaged or is when you shove the piston where it has not been ...the end of the cyl.
How old is the " new kit " you installed
With gaulding , the cyl it can be out of round too
I never would flush bleed brakes on services at Datsun , only clean the tank out . Got in trouble about it ...I said a new cyl is cheap & the way to go . When cold weather hit we had 3 -4 that techs had flush bled .
driving the brakes pushed thru ....no brakes. They stopped flushing on a service

I am watching this thread to see what you found out. I am having the same problem, with new MC, calipers, and brake pads.

Where is the brake pedal in relation to the clutch pedal...even? Does the push rod release all the way when you press or is it stuck?
Linda

The brake master cylinder has a small bleed hole to relieve the pressure when the brake pedal is depressed and released. The bleed hole allows the brake fluid to return to the master cylinder reservoir. If for some reason the bleed hole is plugged, pressure will build up in the brake line with each successive depression of the brake pedal and the brakes will eventually lock.

As Linda asked, is the brake pedal pushrod into the master cylinder fully returning to its normal position? If no, this may be the cause of your brake problem. If yes, the bleed hole may be plugged.

I think the final solution on this , back in 2005 , was I did something wrong and piston wasn't coming back all the way, I remember the bolt holding the piston stripped and I used a bigger one, an I think it covered the bleed hole Believe I replaced the master. Either way, the part Joe posted, for $175.00, is just totally worth it. Even if you get a rebuild kit, who knows how old the rubber is?