Only half of the MC is working???

[AidanDawn2000]

Has anyone experienced only half the twin tank brake MC working? This is for a '69
When I went to bleed my brakes I found that no fluid was coming out the rear bleed screws even with them fully loos. There is no fluid leaking anywhere. In the front everything went perfectly with nice, strong, metered pumps. Got a firm pedal even tho I never actually bled the rears.
I thought that front cup in the MC might be bad (front tank is for rear brakes) but when I tried bleeding the front bleed screw on the MC it worked fine.
Is it possible that something is wrong with the proportioning box? Is it possible there is a blockage on the way to the rear circuit? Seems unlikely given the high pressure but.....

Any and all ideas welcome. Info on cheap aftermarket MCs VERY welcome

Thanks!

-Aidan

[jrusso07]

Proportioning valve is a good guess and a place to start. Also there is a "t on the rear axle to check too. There is a rubber hose that feeds the rear "T" that might be clogged with stuff.

You can pull the Proportioning valve and disassemble it, clean it and reassemble. While your at it flush all the old fluid out of all circuits.

[redroadster]

sure have sounds like the piston seal jammed in the cly at full throw a very common problem
that section of the cly is never used until you open the bleeder
did you disconnect the lines from the M C and see how it pumps ( might want to bench bleed to keep from ruining the paint )
and yes it could have pushed the proportioning valve all the way over , if you have rear pressure open the other cyl and push it back

[tjp]

I had this happen on a "NOS" unit. Turned out part of the master inners were bad out of the box.

[fj20spl311]

FYI, the brake light indicator switch is not a proportioning box or valve.
It is activated when the pressure is higher in one of the circuits and turns on the safety switch lite usually by clipping off the end of the switch that screws into the block. Many people remove the switch from the block before bleeding their brakes. I am not sure if when its "locked" over to one side, it also blocks the fluid flow to the low pressure side, but that sounds like a good idea.

A proportioning valve is really a pressure/volume limiter that is usually adjusted to a pressure just below locking of the rear brakes on a disc/drum system.

[pebbles]

Remove the rear hose at the diff and check for obstructions/degradation. Replace as necessary.
Remove rear wheel cyls and clean thoroughly. Replace as necessary.

[cktrap]

Remove the rear hose at the diff and check for obstructions/degradation. Replace as necessary.

Been there replaced that

[19692000survivor]

I just experienced the same thing this week with my 67 which sat for a very long time. It was all the above mentioned so far.
I started with each rear wheel cylinder and worked my way back to the MC. Collapsed hose at the rear end, brass block where the rubber hose attached plugged too. Both front brake hoses were also collapsed with lines plugged coming in & out of them. Pulled the MC for disassembly and rebuild but for the life of me can’t get the small piston out - any ideas. I have gone through (2) cans of PB Blaster and tried both blowing with compressed air and sucking with a shop vacuum but no luck yet. Is there a trick for removal? Is anything holding it from coming out? Also the big nuts on the bottom are not budging. I am afraid I might break the MC casting trying to remove them - any suggestions? Sorry to tag in on your post but I feel like we are both on the same Pain Train here seeing the same frustrating sights. Back to your questions - I am uncoupling at every point along the brake fluid path verifying all is clear – I predict PB Blaster stock is on the rise, 311s drink it.
Good Luck!
Rick

[mawaru]

I have a similar question to the OP on my 68' which too sat for several years. The MC had fluid low in the front tank (for the rear drums). So I went through the process of bleeding the rear brakes starting with the rear LHD then rear passenger side. While the brake pedal would eventually harden, upon opening the bleed valve in the rear brakes there would only be a drip of brake fluid coming out. On the other hand when the valve was completely open the brake fluid displayed more pressure when spitting out of the valve.

Should I replace the rear wheel cylinders first or rebuild them? Any other suggestions? I also noticed the brake lights are always on now but from what I read on the forums the switch behind the pedal might be bad?

Thanks,
Roland

[ed B]

I have a similar question to the OP on my 68' which too sat for several years. The MC had fluid low in the front tank (for the rear drums). So I went through the process of bleeding the rear brakes starting with the rear LHD then rear passenger side. While the brake pedal would eventually harden, upon opening the bleed valve in the rear brakes there would only be a drip of brake fluid coming out. On the other hand when the valve was completely open the brake fluid displayed more pressure when spitting out of the valve.

Should I replace the rear wheel cylinders first or rebuild them? Any other suggestions? I also noticed the brake lights are always on now but from what I read on the forums the switch behind the pedal might be bad?

Thanks,
Roland

As far as the switch goes, try pressing the switch in and out and see if the lights go off. I believe the switch is adjustible and that may be the only problem. Also you might have to bend the attachment a little so that is is aligned correctly. Mine had a different issue. There is a hole in metal tab that I think accepts a rubber or plastic bumper for the switch. Mine was missing so the switch did not stop in time and therefore was on all the time. I found a nylon bolt at a hardware store and glued it into the hole and that acted as the bumper/spacer and solved the problem for me.

[RustyBucket]

Sounds like my '69 SPL is suffering form the same brake problems. The car has been on jacks in the garage for some time, and new attempt to get it back on the road has begone.

Problems: The brake pedal goes to the floor with no resistance. No lights indicated on the dash.

The rear wheel cylinders were removed, inspected, clean or replaced. brake lines are OK and not leaking, not crushed, brake bleeders operational- not frozen

Front disc calibers, no leaks, hoses OK, metallic lines OK - no leaks, not crushed

M/C was replaced and bleed prior to installation -checks out OK
Indications:- Brake fluid in the Original M/C was BLACK and like thick.

Proportioning box or valve _ WAS NOT REMOVED ( but since reading the DISCUSSION FORUM - its going to CLEANED today!)

New Brake fluid was used to fill the M/C and brake vacuum bleeder attached to the R/R w/c , with the bleeder open - there was NO DRAWING of Brake Fluid despite pumping the pedal.

- W T HECK ?? This is where I stopped and the scratching began... I'm bald now :-@

Suggestions ?? Other than "....put the car on CRAIGSLIST"

- Thanks
Cam

[notoptoy]

Bleed the master cylinder again, on the car. Have lots of rags handy to protect the paint.

[RustyBucket]

I'll give that a shot after I clean out the Proportioning Valve- As for the paint,...it's a red Earl Shibe...Thick and HEAVY!!

I'd like to Soda Blast the WHOLE car ... wield in new metal in the front fenders or get fiber glass fender for the car...that are a pain in the arse ...

Why couldn't Datsun put a inner fender well in there is beyond me
I have considered making a Fiber glass plug mold of my repaired fender and then making a fender with 20oz glass. BULLET PROOF and NO RUST !

[AidanDawn2000]

My problem was that the rubber hose that goes into the brass block on real axle was literally packed with crud. A coat hanger works nicely for cleaning it out tho.

[RustyBucket]

I though of buy a spool of TIG wire from the local Harbor Freight and snaking the line. the Rotary Router deal ...