Datsun Roadster Forum-311s.org

So just to be clear in my mind, you are saying if you take the screw in pin out of the front piston and remove the keeper for the retaining washer on the rod, all of the guts fall out of that M/C? There is nothing to restrict the movement of the rear piston? Is there supposed to be a snap ring in the bore to stop the rear piston from traveling too far? That is just crazy. My 510 M/C has 2 screw in pins that restrict the movement of both pistons. If you get another cylinder make sure you have the correct threads on your brake line connectors. Sorry you are having so many problems

If this is a "driver" buy a BECK/ARNLEY 0722694 7/8" Bore master cylinder from RockAuto You do need to adapt the thread from 10 mm to 3/8 SAE
The cylinder is ~60

Dads my master only has one pin and the location is different It only restricts the movement of the piston marked R (rear.) The snap ring and washer are the only thing holding in the "front" piston.

another view

If you have the snap ring in the back of the master with rod in, then nothing should “fall out” as far as I know. But you do need that freeplay so why not try to measure what play you have with the master installed.
Also check Karp’s Power Brake in Upland for info and pricing on a resleeved master, and they are on the vendor page too.

Linda

Hey Linda, what I was trying to find out is if when he takes the snap ring off the retainer washer is there another stop for the piston. In this case there doesn't appear to be one. So the pedal rod is the stop for the aft piston. Not one M/C I have ever worked on is that way. The piston should not come out if the rod is taken off. No matter what he does he will never get a gap between the piston and rod.
Which brings me to my next question, can he lengthen the rod to stop the piston in a position where it will allow the fluid to return to the reservoir? fj20 what do you think? Is it possible the piston is moving too far back instead not far enough?

"No matter what he does he will never get a gap between the piston and rod." Exactly Mike. Linda, I have had the master in and out 3 times and have tried to establish the gap with several different adjustments. As Mike mentioned above, without a stop for the aft piston, the washer/snap ring/ push rod IS the stop. As a novice it took me awhile to figure this out. It dawned on me this last dis-assembly when I was going to try and grind the ball down to create the gap. The return spring is just going to push the piston to the retainer washer/snap ring anyway. I didn't realize that most M/C's have a stop pin to prevent that until Mikes latest post. This photo shows how the ball end of the pushrod is cradled into the bowl in the piston far enough to eliminate any gap. Grinding it would not gain me any gap. Again in this particular M/C the spring will force the piston to eliminate any attempt to create a gap. I can see how it would work well if the piston had a stop on it as in the photo Mike provided above.

Im interested
Assembled and bench bleeding / bled
The rod should move half inch before the fluid starts pushing out the rear brakes line then the frt.

Got good volume of fluid out both ?
All the things mentioned before frt calipers arent tight ?
Rotors glazed .?..here in the jungle / Midwest when warm humid air moves in hits a cool rotor your brakes seem 50% effective until warmed up
The brake switch valve may be out of center
( if say the left front B line sprang a leak the piston blocks. bbqthat line so the next pump gives the other brake operation , this can happen bleeding brakes , best to pump fluid thru with the bleeders open

Also, check that the brake pads slide in and out of the caliper housing freely. Apparently some are a bit "thick" and are tough to install - that equates to not retracting properly as well. BTW, what about the rubber hoses? They have been known to fail internally and act as a check valve.

I don’t understand the “ gap at the ball end”, I never knew it existed or was supposed to. All I thought was important was the push rod stays in. For that you have the snap ring, pretty effective, and then just connecting the rod with the bracket to the pedal. Then you have some” play“ which is when you are pressing the pedal but the rod is not doing anything yet. If the push rod is too long , as soon as you touch the pedal, the rod “is doing something”. That is not supposed to happen so, as I see it, you cut the push rod to get some “play” on the aftermarket adjustables. OEM is one size fits all or you use some firewall shims- very inefficient
i think initially my push rod was so long it hit the pedal. Cutting it established the “ play”..(.movement that doesn’t start to push the piston yet..)
Not sure you have that, or enough “ play” to be fully effective.
But the other issues mentioned seem like they could be a factor...sticking pads as things heat up. Bulging brake hoses. Air in the system. Etc.
Also I don’t see where you said that you bled the master yet, maybe I missed it. You can put a speed bleeder on the master too, not sure of size, then clear tubing on the bleeder and back into the reservoir. Hold it on and have your assistant pump...should be no bubbles.
Linda

As have been pointed out before. The important fact is the rear port needs to be open.
Sorry for the crude drawing, but if the piston does not clear the port, you could bevel the pushrod where it contacts the beveled washer.

I like the idea of bench bleeding the cylinder. Not likely to be the hoses as I think you said it happens on both sides, but they might be next.
With the MC assembled, you can use a squirt bottle to check if the route from the metal line to the reservoir is clear.

Yes, The master has been bleed, several times. Brake pads have been sanded on the edges to assure free movement. Tire/wheel assembly spins freely with frt end jacked up when brakes are cooled down, indicating that caliper assembly is correctly centered over rotor.

Time to check your "master cylinder residual valves".

There is a master cylinder residual valves under the large nuts. Its possible that its sticking or someone swapped the rear spring for the front.
In theory the rear spring should hold 10 psi and the front should hold 2 psi. Usually they are so weak by now that it make no difference.
If the valve is sticking, it could cause a residual pressure that would blead off with time. The valve (spring and plunger) can be removed, but the system works better with them.

With all the subtle possibilities and the fact that I have a master of unknown origin that is obviously not an O.E. type master complicating the diagnostic process, it seems replacing the master might be my next best move. There is the possibility that it might not solve this ongoing problem, but at least it will eliminate one possibility. And i'll have the comfort of knowing that I have a new master. I'm not in the position to just throw money at the problem but im frustrated to the point that i'm willing to spend the dollars. I cant tell all of you how much I appreciate your input and insights. I have learned a lot about my little car in this process, all good valuable experience.

One of the suggestions by several members was to be certain that the caliper assemblies were centered over the rotor. I did a visual inspection that seemed to my eye to look like they were centered. Also the rotors spun freely with no rubbing. I mentally eliminated that possibility as the problem. Since I am going to have to wait for a new master to arrive, I am once again out in the garage tinkering and trying to see if I have overlooked anything. This morning I removed the tire/wheel assemblies and the retaining clip for the brake pads to double check that they are not binding and prohibiting free travel. Since I was there anyway, I bought a feeler gauge and tested clearance on each pad. The difference in clearance from the outboard pad to the inboard pad is .013 inches or .330 mm. Could this be enough to cause the overheating problems ive been chasing? If so, where in the hell am I gonna find a shim that thin 800 miles from the U.S. border? YIKES!