New Master Cylinder Defective?

[Cool67]

I recently put in a brand new master cylinder in my 67 Mustang. I first bench bled it and that went OK. I then connected all the brake lines and gravity bled the system. That also went OK. I had a leak in one of the metal connections so I tightened it and then pressed the brake pedal a few times to see if there were any more leaks (before bleeding the system). The leak stopped and then I gravity bled the system again. But this time, there was no fluid coming out of the rear brake lines. I left the rear bleed plug open and pressed the brake pedal several times and no brake fluid came out. So, I assummed that when I pressed the brake pedal the first time, it must've moved the plunger in the distribution block and cut off the fluid to the rear brake lines. I then bought new fittiings and bypassed the distribution block for the rear brake lines only. I tried bleeding the rear brakes again and still no fluid. I finally removed the brake lines from the master cylinder (rear brakes connections only) and pressed the brake pedal. Still no fluid out of the rear connector (with the resorvoir full) of the master cylinder. It seems like this brand new master cylinder (from NAPA) is defective.

I could put back the original MC but the fittings were different and it would take a bit of work to put everything back the way it originally was (long story).

I plan on returning it but would like to see if someone concurs with my reasoning and/or get some explanation of why this happened or if I did something wrong in the process.

 

[red65]

If you left the bleed valves open while you were pumping the brakes (forgive me if this isn't what you were doing - it's what your post seems to say), then you were drawing air into the lines every time you let the pedal back up.

1 Push down on brake pedal
2 Crack open bleeder screws (1/4 turn or so) while brake pedal remains depressed
3 Close bleeder screws (brake pedal still depressed)
4 Release brake pedal

 

[jcode68]

I would try bench bleeding the MC again to be sure you didn't pull air back into the line. If that fails, then return it.

 

[Rusty67]

I agree with jcode.

 

[Cool67]

If you left the bleed valves open while you were pumping the brakes (forgive me if this isn't what you were doing - it's what your post seems to say), then you were drawing air into the lines every time you let the pedal back up.

1 Push down on brake pedal
2 Crack open bleeder screws (1/4 turn or so) while brake pedal remains depressed
3 Close bleeder screws (brake pedal still depressed)
4 Release brake pedal

When I first pumped the brakes, all the bleed valves were closed. After that, I tried everything I could think of and still no fluid out of the rear port of the MC.

 

[Cool67]

I would try bench bleeding the MC again to be sure you didn't pull air back into the line. If that fails, then return it.

I did another "bench bleed" last night while the MC was still on the car. I could see one or two bubbles in the rear brake reservoir (with the bleed tube returned to the reservoir) but no fluid movement . The front brake reservoir had a lot of fluid movement.

(Actually, I found that doing a "bench bleed" with the MC on the car was a lot easier and less messy than doing it on the bench with a vise.)

 

[Rusty67]

When you are bench bleeding in the car make sure to jack up the rear end so that the rear bowl in the MC is as high as the front bowl. The MCs are angled up a little bit and wont bleed right unless you jack the rear to even it out.

 

[Cool67]

When you are bench bleeding in the car make sure to jack up the rear end so that the rear bowl in the MC is as high as the front bowl. The MCs are angled up a little bit and wont bleed right unless you jack the rear to even it out.

OK. Thanks for the tip.

 

[Cool67]

Still need help with bleeding the MC

I returned and got another brand new MC but I still have the same problem. The reservoir for the rear brakes doesn't seem to be bleeding correctly. It almost seems as though the plunger is not going back far enough. I'm still bleeding the MC on the car but the brake pedal doesn't go all the way down to the carpet (about an inch away). I'm not sure what's stopping it. Maybe that's the reason you have to bleed it on the bench first.

I did level the MC as Rusty67 suggested but even after pumping the brake pedal a hundred times, I still don't get any fluid motion in the reservoir for the rear brakes (with the bleed kit attached).

I know it's something I'm not doing right because two brand new MC cannot be bad.

The bleed instructions that came with the MC says that you have to slowly depress the MC piston 3/4 to 1" (I know the brake pedal moves more than an inch) until the piston cannot be depressed more than 1/8 inch and no air bubbles are visible. I would think that with the bleed kit hoses looping back to the reservoir, it's still an open line and no pressure would build up.

Can anyone confirm that the piston actually gets hard to push after a while?

Anyone have any suggestions on what I can do?

 

[Rusty67]

I don't like the bleeder tube deals. I use the plastic plugs to bench bleed the MC. They screw into the MC ports and have always worked well for me.

 

[jcode68]

When bleeding the MC (on the bench or in the car) you will not get a firm pedal as there is no pressure build up. You are done bleeding the MC when there are no air bubbles. Reading your last post, sounded like you wanted confirmation of this. Also, if your pedal is not allowing the MC to fully engage, then you need to adjust the MC rod by lengthening it. Most that I have seen have an adjustable end on it so that you can fine tune the pedal engagement point. You shouldn't need much, maybe 1/8 to 1/4 inch. How do you know you are not getting fluid motion on the rear brake portion of the resevior? It will not be obvious once the air is bled from the MC. I would adjust the rod length, do another final round of bleeding and then connect the lines and bleed the wheel cylinders and see what happens.

 

[Cool67]

When bleeding the MC (on the bench or in the car) you will not get a firm pedal as there is no pressure build up. You are done bleeding the MC when there are no air bubbles. Reading your last post, sounded like you wanted confirmation of this. Also, if your pedal is not allowing the MC to fully engage, then you need to adjust the MC rod by lengthening it. Most that I have seen have an adjustable end on it so that you can fine tune the pedal engagement point. You shouldn't need much, maybe 1/8 to 1/4 inch. How do you know you are not getting fluid motion on the rear brake portion of the resevior? It will not be obvious once the air is bled from the MC. I would adjust the rod length, do another final round of bleeding and then connect the lines and bleed the wheel cylinders and see what happens.

Thanks for the confirmation about the pedal pressure. I assumed that there was no fluid motion on the rear brake reservoir because 1) I could see a lot of fluid motion on the front brake reservoir, 2) when I disconnected both bleed hoses, brake fluid dripped out of only the front reservoir.

I didn't notice any adjustments on the push rod but I will take a look at it again (I'm hoping they just pull out once they've been inserted). If by chance it's not adjustable, what else can I do? Do you know what keeps the brake pedal from going all the way down to the floor?

 

[Cool67]

I don't like the bleeder tube deals. I use the plastic plugs to bench bleed the MC. They screw into the MC ports and have always worked well for me.

The bleeder kit I bought from NAPA didn't have any plastic plugs. Where did you get yours?

 

[Rusty67]

When I buy parts I buy A1/Cardone for MCs. They come with them in the box. Usually a blue on and a yellow one... If I remember right.

 

[jcode68]

Cool67, how did this story end?