Master cylinder not pumping fluid to front brakes

[bchen]

I recently bought my 1970 roadster.

I knew the brakes needed work when I bought it. I was able to bleed the rear brakes, but nothing comes out of either front brake bleeder. I then disconnected the front brake line at the brake distribution / brake junction block (photo below).

IMG_0553.jpeg

Pumping the brakes didn't make anything come out of that connection either.

I bought a replacement dual master cylinder from SinCityDatsun, swapped that in, and bled the master cylinder in place: installed master cylinder, attached tubes from the bleeder valves on the sides that emptied back into the cylinder, pumped brakes until fluid was bubble free. Strangely, only the rear reservoir drained. The reservoir closer to the firewall that controls the front brakes never pumped any fluid out of its bleeder valve. Fluid level on that front reservoir also did not drop.

I'm scratching my head...

Any ideas / advice on what's going on and what I should try next?

Thanks!

Bernard

[redroadster]

Sure you pressed it fully
It pressurized the rear brakes 50-70 % before the frt brake side starts flowing
Possibly the seal stuck at the bottom of travel
I've connected a line from rears to front and pressured it loose , otherwise they sent a defective part
Some remans place cosmoline or something very close to it that is best to use naphtha, acetone to free it up , the old VW stuff noted for this

[Habitat.pat]

Bernard, if you just loosen the bleed on the front wheel master cylinder does any fluid leak out? It should. If not, try removing the bleed screw completely.

Peace,
Pat

[unklpat]

Also, you might want to remove the caliper bleeders, and the steel balls. They could be rusted in place. Speed bleeder part #3824HD will be your new best friend. Trust me. Pat

[Solex68]

I'd contact SinCityDatsun as they are great to work with and super knowledgeable. I'm sure they help you figure it out.

[bchen]

Bernard, if you just loosen the bleed on the front wheel master cylinder does any fluid leak out? It should. If not, try removing the bleed screw completely.

Peace,
Pat

I tried what Pat suggested - removed the bleeder valve, pumped the brakes, and no fluid came out! Educational. There's something about the new master cylinder.

So I took apart the old master cylinder, made sure there was no pitting, cleaned the parts, reassembled it, and installed it. That worked! Bled all four lines. Also bled the clutch fluid while I was in the mood.

Will try to drive the car tomorrow!

[Loy]

I had this same issue. The rears would bleed fine. I had to “bench bleed” it first for it to work per the instructions. DOH! It took a while to figure this out as it was unlike any other master cylinder I’ve done before. After doing this everything worked…

[jasmith1975]

Also, you might want to remove the caliper bleeders, and the steel balls. They could be rusted in place. Speed bleeder part #3824HD will be your new best friend. Trust me. Pat

Do you know if the Dorman 12701 Speed Bleeders will work? Goodridge are twice the cost and a longer lead time. Of course I waited to order and need them yesterday.

I had this same issue. The rears would bleed fine. I had to “bench bleed” it first for it to work per the instructions. DOH! It took a while to figure this out as it was unlike any other master cylinder I’ve done before. After doing this everything worked…

I have an issue where the brake pedal goes to the floor (brake dash light came on). Thankfully I noticed before driving, but couldn't find any leaks. Puzzling. I decided to change out the rubber hoses as they looked original and were pretty dry rotted. That's done (finally) and I'm ready to bleed the master cylinder (on the car) and then the brake lines. Any insight how to go about tackling the M/C as there's no brake fluid in the lines or reservoirs? I've only bled brake lines before, not a M/C, and appreciate any tips.

[redroadster]

Get the MC pumping with both lines unhooked
They make clear vinyl tubing to do that or a steel one if you find the right thread ,with the lines going back to the MC reservoir and pump until no bubbles , sounds like you un centered the safety switch , once you get a firm pedal it may center or you let the opposite side barely open and push/ center it back u til you do fluid won't flow on closed side

[redroadster]

Get th. Duplicate

[jasmith1975]

Get the MC pumping with both lines unhooked
They make clear vinyl tubing to do that or a steel one if you find the right thread ,with the lines going back to the MC reservoir and pump until no bubbles , sounds like you un centered the safety switch , once you get a firm pedal it may center or you let the opposite side barely open and push/ center it back u til you do fluid won't flow on closed side

Thanks for the advice. Hoping you can help with a few more noob questions.

- when bleeding the MC, can I leave brake lines connected and hook up hoses to the bleeder valves with other ends going back into resevoir?

- re: safety switch, is that the proportioning valve? What do you mean by "let the opposite side barely open and push/center it back"? I haven't look too closely at that and it may make sense after I do.

[DatsunNoob]

jasmith1975 said: "- when bleeding the MC, can I leave brake lines connected and hook up hoses to the bleeder valves with other ends going back into resevoir?"

Short answer is 'yes'.....long and correct answer is 'NO'. You need to get the M/C cleared of all air and the piston centred front to rear. That's what you will accomplish doing just the M/C first. Then, reconnect the brake lines and bleed all 4 wheels, right rear, left rear, right front, left front - longest run to shortest run of the brake lines.

The reason you don't want to try and run lines back to the m/c from the bleeder valves is that bleeding may introduce air into the system and that's a really, really long run of the drainage system to get it all out. It MIGHT work but personally if I did it, it would only be an experiment to see if it works and I would properly bleed the system before driving the car.

[redroadster]

Get the MC BLED fully 1st ( before hooking up brake lines to it ,the easyest way by far )
You almost always have no air in the lines ,if you had good brakes , trying to bleed thru the calipers with air in the MC , can cause the safety switch to un center, which many times is a pain to get back

[jasmith1975]

jasmith1975 said: "- when bleeding the MC, can I leave brake lines connected and hook up hoses to the bleeder valves with other ends going back into resevoir?"

Short answer is 'yes'.....long and correct answer is 'NO'. You need to get the M/C cleared of all air and the piston centred front to rear. That's what you will accomplish doing just the M/C first. Then, reconnect the brake lines and bleed all 4 wheels, right rear, left rear, right front, left front - longest run to shortest run of the brake lines.

The reason you don't want to try and run lines back to the m/c from the bleeder valves is that bleeding may introduce air into the system and that's a really, really long run of the drainage system to get it all out. It MIGHT work but personally if I did it, it would only be an experiment to see if it works and I would properly bleed the system before driving the car.

Thanks for the tip. I disconnected the brake lines at the MC and will bleed that first. Just need to grab a bleeder kit and get to work. To clarify my earlier question, I was asking about hooking up hoses to the bleeder valves on side of the MC and running them back to resevoirs. I might have to buy stock in clear tubing to run them from each wheel.

Get the MC BLED fully 1st ( before hooking up brake lines to it ,the easyest way by far )
You almost always have no air in the lines ,if you had good brakes , trying to bleed thru the calipers with air in the MC , can cause the safety switch to un center, which many times is a pain to get back

Brakes are good, but pedal went to the floor after sitting for several months. Hoping it was just a dry rotted hose. Bleeding MC then brake lines will be the test. Also hoping the rear hose never has to be changed again in my lifetime. Lol

[jasmith1975]

Update: I disconnected the brake lines from the MC and installed hose barbs and hoses, routing them back inside the reservoirs (MC still on the car, btw). Filled reservoirs with brake fluid and slowly (fully) pumped the pedal a few times. Rear seems to cycle fluid through the hose but not seeing the same for the front. With the front, I see fluid start to move up the hose when depressing the pedal but then stops and goes backward as the pedal is further depressed.

I snugged up the hose barb and confirmed the hose was firmly pushed on the barb with no kinks in the hose.

Is this a sign of a bad MC or am I doing something wrong? Some earlier posts mentioned having the same problem and resolved with bench bleeding or disassembling/cleaning/reassembling. Btw, side MC bleeder valves are fully closed.

Appreciate the help!