Datsun Roadster Forum-311s.org

They say bad things come in threes....here's a sad story that supports that.

1970 SPL losing brake fluid at a low rate. Pull the wheels and drums, driver rear cylinder leaking...replaced the cylinder and bled. Brakes are good. Now, leaking from the master inside the firewall. Ugh

1970 SRL - stains on my brake/accelerator right sneaker...hydraulic fluid leaking from brake master inside fire wall. Ugh. Refilled (topped off) the master. Seems to be holding. Brakes well. Cleaned my sneaker.

1969 SRL - steel braded fuel line broke free of bracket and rubbed on brake line, it punches through the line and (damn!) no brakes! (front line from master to proportioning valve). Replaced line, reinforced fuel line bracket, bled brakes...all good. Today I went to the barn to work on the SPL and the master cylinder on this car is empty. New brake lines and fittings are dry...brake fluid on the rubber floor mat! Leaking from master inside fire wall. How can that be?

I am cursed.

All sharp objects have been removed from the workshop. No ropes or chains either. I have a garlic rope on each master.

So the tech question are, 1) will the pumping of the master cylinder to bleed brakes cause a seal failure on the pushrod side of the master? If so, is this a temporary condition that will right itself with some normal use? 2) Will sitting unused cause a leak at the pushrod seal? I have had these cars for over 10 years and never had any hydraulic problems. I guess I must be due?

The other two master cylinders haven't lost any fluid and they brake well. I just refilled and bled the 69...we'll see how she fairs in a few days.

There isn’t anything I know of in the master that would cause this, but when pumping the pedal to bleed the system you are pushing the piston farther into the cylinder than normal. If there is rust or cruft in there it could tear up the seals.

Peace,
Pat

A well used master cyl ...when pushed to the end of it's travel with no pressure stopping will often go out soon after ....reason why is
The steel piston has been sliding on the cylinder and shedding sharp flakes of steel into the fluid side where they stay the fluid travel has been stopping mid way in the piston travel but with no pressure it goes to the end and those flakes imbed in the seal , also bit of the rubber seal
also the seals tend to balloon
With pressure keeping then inflated but no pressure let's them deflate with out much of a notice things are wrong ...if you.live in a hilly area always best to go with a new master cyl
Most shops will not flush brake fluid anymore because of it ...like it was in 60s- 76
And when the weather drops into the 50s the seal stiffen and leak easy

Pat and red - both explanations seem accurate. I did flush the fluid on both the 1970 and the 1969 just because I was bleeding the system and wanted to have clean fluid. I do live in a hilly area so new master cylinders is the order.

Thanks!

One new brake master installed. Bench bled and then mounted and lines connected. But what a PITA, brake fluid running all over as I tighten the line fittings 18 degrees at a time due to constrained area. It doesn’t help when you have catcher mits for hands…

Yup, it sucks.

I'm fighting the brakes on mine rn too...so happy I went with dot 5 though with the leaks -_-

Silicone brake fluid mixed with just a bit of dot 3 ...gels and pretty quick in summer
When it came out I tried it on my mitsu on the brake I put brake kleen in and flushed it out ,I thought ...but 2-3 months later it gelled in the safety valve / brake lite swt.
Dot 5 is to have neoprene seals
You don't know cursed til you take a 90s GM 4
piston ABS system to a fake / in competent dealer for a master cyl replace