Master Cylinder Bleed Question

[JamesDean]

Lets say I have my master cylinder (500E with 500E booster) installed in the car...my 93 190E.

I hook up a master cylinder bleed kit. The kit has clear lines so I can see air bubbles.

I have someone pump the brakes in the car.

The line closest to the brake booster (side output) sees fluid movement first. A few pumps and the air bubbles are gone. I can still see fluid movement as my buddy pumps the pedal.

The line furthest away from the brake booster (top output), I don't see fluid movement until my buddy gets to >75% pedal position and it takes a good amount of pumps to clear air out.

My questions:
1) Which line supplies the front caliper pair? I suspect front calipers are on the top output line.

2) Why does it take >75% brake pedal travel to make the top output push fluid? This does not seem right to me. What adjustments or changes can be made to rectify this?

Looking forward to thoughts on this...

[tecqboy]

I'd like to know as well. I installed a new MC on my 91SL and even after several bleedings with a positive pressure bleeder, I still have what I feel is excessive pedal travel before the front brakes begin to grab. I thought that the large piston / cylinder toward the rear of the MC was for the front brakes. I have no air bubbles, and my brakes work marginally OK, but I have to push on the pedal almost halfway before there is any resistance. I know this isn't right, but I can't find any cause for it either. Does the ABS box need some special attention when bleeding? What effect does the vacuum booster have on the pedal travel? I'd surely feel better if the pedal was firm right at the top.

[Hirnbeiss]

it should be a split, with each port supplying one front and one back. Once you have bled the MC, the bubbles should not return. If they do, a leaky seal is sucking air in somewhere - maybe the reservoir grommets. Once both cylinder halves are full of fluid, they should discharge simultaneously.

[JamesDean]

Alright, so let's assume the bubbles have been dispatched and I'm just observing fluid movement.

When the brake pedal is pressed, I see no movement in the top port(furthest from the brake booster) until the pedal is >50% of travel. This is on a new master cylinder, mated to its matching brake booster. What would explain this?

[David Wilson]

OK, as I see it, the front calipers hold more fluid. That's a given.
The front calipers also require more fluid movement than the rears due to the larger piston volume.
So, with that in mind, the MC is designed to send more fluid to the front than the rear.
I assume that there are twin chambers of different sizes that the MC piston operates in. It must have two different size piston seals on the piston shaft.
Therefore, you will see more movement of the fluid to the front brakes.
There may also be a bias built-in to the MC design to delay the fluid one way or the other.
As far as fighting the air battle goes, back in the day when brakes were simple, pressure bleeding with the pedal worked well. Now days you need a pressure tank bleeder, or a vacuum bleeder. Too much tubing and other devices for air to get caught in.

[JamesDean]

David,

I'm seeing more fluid movement in the rear port (side output) of the master cylinder (closest to cabin). Very little in the front port (top output)...

Based on what I saw during the bleeding procedure...if the top outputting port powered the front brakes....that explains why I dont get really good braking until I'm down >50% on the pedal.

[David Wilson]

Is your MC matched to your calipers? The OE MC does not move enough fluid to power the big calipers. Thus you will have a lot of pedal travel.

[JamesDean]

This I'm not sure of. Is the pushrod adjustable? I did not think it was.

[Zulfiqar]

I believe the farthest out port in the master cylinder has a letter of H on it - that is rear brake circuit. The front brake circuit is marked as V

V = vorne - front
H = hinten (sp) - rear

[Hirnbeiss]

I have to come back to the original post: why did you do an MC bleed with the MC in the car? Usu this is done on the bench with a new unit to reduce the time bleeding the lines once it's installed.

Do a thorough bleed at each caliper until no bubbles are present at any of them. This may take 1/2 to 1 quart of fluid. Then you should have a firm pedal. If you suspect your brake hoses are old and should be replaced, replace them first.

[JamesDean]

Originally Posted by Hirnbeiss

I have to come back to the original post: why did you do an MC bleed with the MC in the car? Usu this is done on the bench with a new unit to reduce the time bleeding the lines once it's installed.

Do a thorough bleed at each caliper until no bubbles are present at any of them. This may take 1/2 to 1 quart of fluid. Then you should have a firm pedal. If you suspect your brake hoses are old and should be replaced, replace them first.

Is there a problem doing it on the car? I can't see a problem bench vs car.

I don't suspect the hoses. They're relatively new.

[compu_85]

The problem with doing it on the car is that depending on the mounting angle you might not be able to get all the air out. Easier to do on the bench.

The reason you didn't see fluid coming out of the back port until lots of pedal movement is because you were having to bottom out the forward piston into the back one to get it to move. Normally the system is sealed and the brake fluid will do that for you.

-J

[Zulfiqar]

shouldnt the farthest port be beneficial with a slanted master cylinder as air wants to rise to the top and that is the top of the cylinder when mounted.

[Hirnbeiss]

I guess my question is "why did you bleed the MC" (was it a new one, or did you have some sponginess you were troubleshooting?). If it's new, you may not have gotten all the air out of one of the ports - bench bleeding can take a long time sometimes; and if it's old, the MC seals could be kaputt.

Either way, you'll need to bleed the lines once you open the system.