Help - Bad master cylinder , booster or both???

[jwldane]

Hi everyone,

I thought I had this figured out and narrowed down to master cylinder.....but......

The brakes have been bad since I bought the car. Since I have been rebuilding and re-engineering the interior it hasn't been much of an issue as it doesn't see the road that often. Ah, but then came the warm spell after winter and I had to her out for a spin. Now the brake pedal has always gone pretty much to the floor with very little effort but would effectively brake at the end of its travel. It was okay once you got used to it. I could hear a hiss when I stepped on the pedal and at first thought the booster was bad, but the braking was not difficult. They acted as they we getting the assist from the booster, no need to break the seat back stomping on the pedal. I did the standard booster check, pump the pedal with car off, then start car and pedal gently goes back to floor, all appears okay. So now I'm thinking its gotta be a leaky seal in the master. Cheap and easy enough fix.

Now back to the first drive of the season. Brakes are all of a sudden much worse. Still stop the car without a tremendous effort but with a lot less certainty or feel. I would not be comfortable taking it out anywhere beside right around the house. I am now also getting a varying idle speed when a press the pedal. If I remember correctly, the idle gets faster when the pedal is depressed. Now that leads me to think vacuum leak and the booster again. I do not have any leaking brake fluid and the brake reservoir has the correct amount of fluid.

That brings me back to the subject line. Master cylinder, booster or both? Any help or suggestions greatly appreciated.

Thanks
John

[vandor]

Likely both. If it goes to the floor then one of the circuits is not working. Likely the rear if the car appears to be stopping ok.
When both circuits are working and properly bled the pedal will not go to the floor, unless something is badly misadjusted.

[jwldane]

Thanks for the response.

I was afraid it might be both. Changing the booster out is a much bigger job. After reading more about boosters here, I'm a little nervous about proper stud length and rod length adjustment. I saw that AR has a high performance booster/MC combo. Do you know anything about that? Would it really offer that much improved performance for 3 times the price and do they come pre-assembled and adjusted?

I believe the system if functioning on all four wheels. All the rotors are bright and shine like the pads are making contact and I don't notice any rolling resistance except for when I forget to disengage the e-brake.

The thought of wrestling under the dash disconnecting springs and cables is making my arthritis flare up already. I may just try to replace master cylinder first with the vacuum and fluid lines and hope for the best. I don't think the booster is that old but I wish I knew how to determine with certainty that's its bad before I start pulling the seats out and crawling under the dash.

Thanks again...

[azruss]

I am just finishing up doing the same thing on my 80. You will need to pull the fuel rail to get the booster out. Good time to get your injectors rebuilt. The rod length adjustment can be easily accomplished with a dial caliper. The rod should stick out from the booster bolt face .041-.049". I used the edge of a feeler gauge across the hole and measured from the edge of the blade to the end of the rod with the depth part of the caliper and did the math to determine the amount sticking out.
I agree, disconnecting the brake pedal and unbolting the booster is hard on the old shoulders and hands.
I am having trouble getting the brake lines to not leak at the MC. Not looking forward to solving that issue.

[vandor]

The hi-performance bosster and MC come pre assembled, however it does not sound like you need it.
Doing the pushrod adjustment on the standard booster is not hard, and most people don't even have to adjust it, just check it.

[jwldane]

Thanks again for the replies.

Did another booster check. Pedal holds firm and steady when pumped while car is off. Slowly moves down when engine started and quickly firms back up after turning engine off. I'm betting booster is okay. Fingers crossed.

Going to order and replace MC, reservoir and lines, the vacuum line and check valve to booster. Switching to single reservoir rather than the two separate small ones I have now and add a warning light cap. Trying to get crud out of the one side of the used replacement reservoir. Soaking and shaking with new brake fluid is doing some good but its slow going.

Sorry I wasn't clear about the actual car I'm working on. This is the single carb car built from 9 different cars from 71-82. Our other is an 80 FI as is the parts car.

Besides the seals on the MC, I think the lines to the reservoir are shot. Is there any significant advantage to using silicone lines rather than the standard braided stuff?

Thanks.

[Rambo]

Hi everyone,

I thought I had this figured out and narrowed down to master cylinder.....but......

The brakes have been bad since I bought the car. Since I have been rebuilding and re-engineering the interior it hasn't been much of an issue as it doesn't see the road that often. Ah, but then came the warm spell after winter and I had to her out for a spin. Now the brake pedal has always gone pretty much to the floor with very little effort but would effectively brake at the end of its travel. It was okay once you got used to it. I could hear a hiss when I stepped on the pedal and at first thought the booster was bad, but the braking was not difficult. They acted as they we getting the assist from the booster, no need to break the seat back stomping on the pedal. I did the standard booster check, pump the pedal with car off, then start car and pedal gently goes back to floor, all appears okay. So now I'm thinking its gotta be a leaky seal in the master. Cheap and easy enough fix.

Now back to the first drive of the season. Brakes are all of a sudden much worse. Still stop the car without a tremendous effort but with a lot less certainty or feel. I would not be comfortable taking it out anywhere beside right around the house. I am now also getting a varying idle speed when a press the pedal. If I remember correctly, the idle gets faster when the pedal is depressed. Now that leads me to think vacuum leak and the booster again. I do not have any leaking brake fluid and the brake reservoir has the correct amount of fluid.

That brings me back to the subject line. Master cylinder, booster or both? Any help or suggestions greatly appreciated.

Thanks
John

Varying idle speed and hissing upon pressing the brake pedal is a sure sign that you have a vaccum leak in the brake booster. That is exactly how mine failed.

[phaetn]

Varying idle speed and hissing upon pressing the brake pedal is a sure sign that you have a vaccum leak in the brake booster. That is exactly how mine failed.

That is exactly what happened to me at the end of last driving season and I am going about addressing it now.

So is the solution to buy a whole new brake booster is it possibly a simple seal that doesn't need the whole thing to be replaced? I know I don't want to have to open it up...

Thanks and cheers,
phaetn

[Rambo]

Varying idle speed and hissing upon pressing the brake pedal is a sure sign that you have a vaccum leak in the brake booster. That is exactly how mine failed.

That is exactly what happened to me at the end of last driving season and I am going about addressing it now.

So is the solution to buy a whole new brake booster is it possibly a simple seal that doesn't need the whole thing to be replaced? I know I don't want to have to open it up...

Thanks and cheers,
phaetn

I have seen instructions for rebuilding a brake booster in the manuals but the booster is a sealed unit which would require complete disassembly of the internal parts. I dont think they even sell a booster rebuild kit so I'm afraid the only solution is to replace the booster.

[jwldane]

UPDATE!!!

Replaced master cylinder. Changed reservoir to newer style with idiot warning light switch, new lines from reservoir to MC, new check valve of booster with new vacuum line to intake manifold. Bled brakes (with much help from contributors to this site ie., keep weight on rear axles) and brakes are better......not perfect. Still way too much pedal travel before brakes really engage, but when they do the car stops fine and does not require excessive pedal effort. I would guess that about 2/3's on the pedal travel is without effect with the final 1/3 doing some actual braking. Going to do another bleed and re-check. Booster still seems to check out okay with all the standard checks.

Question..... how much excess pedal travel should be expected if the push rod from the booster is too short? I'm certain the rod isn't making contact with the MC when the brakes are not engaged, but could a couple of millimeters account for the excessive pedal travel? What else could I be missing?

Thanks

[vandor]

Yes, a couple of mm could make a big difference.
I believe the spec is that the pushrod should stick out 1 mm past the surface of the booster.