Brake Pedal Stiff after reman booster and 3 reman calipers - PeachParts Mercedes-Benz Forum

proudfoot · #1 08-09-2011, 01:15 PM

I'm at my wits end here folks and am stumped. Here's the story. I parked my '84 300SD last year for 3 months while out of town and the brakes worked wonderfully before I parked it. 3 months later I go to move it somewhere else and the brakes are very stiff. I let it go for a while and now I am taking care of it. It is a california car with NO rust on it. Never had a stuck bolt, no rust anywhere under the body.

1. I decide to replace the fluid first and see if that helps. In the process the bleeder screw snaps off on the one of the front calipers. Didn't want to mess with drilling and tapping and this was a caliper I had found in a junkyard after the original had a seized piston so I get a remanufactured one. Looks good, pistons move, new pads, good to go. The other front caliper moves freely so I left it with new pads. Bleed and put back together everything, still stiff.
2. I decided to bite the bullet and get a brake booster. Went to advance auto and got a remanufacted booster (this comes with a new seal and the old booster didn't look as if any fluid had accumulated in it). I put it in and brakes are still stiff after a thorough and uneventful bleed.
3. I check the back calipers and the pads are shot and one caliper has a stuck piston. So I bought two remanufactured calipers for the back along with new rubber lines and pads, bleed the whole thing, and the pedal is still stiff as all get out.
- I have very recently rebuilt the vacuum pump on this car and get a solid, needle jumps right to 20hg, amount of pressure at the booster. When I shut the engine off and crack the vacuum line there is still pressure in it at least 10-30 seconds after shutoff. Haven't tested longer. I hooked the master cylinder and booster together with NO brake lines attached to the cylinder and pressed on the pushrod, a healthy amount of resistance to push both booster and cylinder pistons in. Then I turned the car on (with all vacuum lines plugged except to the booster (even when I have all the vacuum lines connected I still get 20hg at the booster)) and pressed it again. Less resistance when pushing. It's doing something, just not when I have the brake lines bled and attached. The master cylinder seems to be in good working order, I have not noticed any fluid loss or mushiness in it, when bleeding I get good pressure and amounts of fluid traveling to the brakes, there seem to be no obstructions.
There are only two things I can think of that would be causing this.
1. There is an adjustment I need to make, or a bushing I need to add to the booster when it is hooked up to the pedal to engage it at the right position. (That is hard to explain, I hope you get the jist. I've seen a bushing, a cylindrical ferrule that is apparently used on some boosters.) This seems the most likely but I can't find any info on it.
2. My pads are sticking. These seems less likely, although I was VERY surprised at the amount of resistance I had to overcome to get the pins past the retainer clips when putting the pads on. On all but one of the remanufactured bendix calipers with the wire retainer clips, I had to forcefully push on the pins and tap them in with a hammer. When I've changed pads before on this car, I never had that much resistance. Maybe they wear in after time. The reason this seems less plausible is that I greased everything and it's not like the pads have to travel that far. They are not rusted or stuck in.

The master cylinder piston moves freely and squirts lovely amounts of brake fluid everywhere when I bench bleed it. When I bench bleed, I press the piston in, then cover the holes with my fingers so as not to allow air back in. I do this until I get good amounts of fluid through each one with little burping. Then I hook it up, and bleed first at the fitting on the master cylinder, and then go to the corresponding bleeder in back, starting with the rear right and then the left. All I can say is, there is NO mushiness in the pedal when I am done bleeding the brakes. I'm going to put everything back together again today and hope that I had just done something wrong last time.

I have also rebuilt the front suspension (got the kit from ************** with all the bushings and what not, rides like a dream), the injectors and pop-tested them, the turbocharger, and put new flex disks in the drive shaft. I'm a shadetree mechanic and enjoy it, I am just stumped right now and can't move to Indiana from New Orleans until I get this fixed. I'm in Mercedes Benz brake purgatory here and need some help. Thanks for any and all help!

proudfoot

connerm · #2 08-09-2011, 01:21 PM

With all of the brake work you did, why didn't you also replace the brake hoses? They deteriorate on the inside and can fail causing something like a check valve. I expect this is your problem.

barry123400 · #3 08-09-2011, 01:41 PM

Drop a pm to Mr.Hunter. I myself do not have a clue of what is going on there. Sitting just three months does not sound like enough time to bind up all the calipers.Especially in your climate. Yet you have eliminated even that possibility.

The only thing I can think of otherwise is that if the car sat outside. Is there any possibility someone squeezed the metal brake lines and tweeked them so no pressure can get through the lines? Or only with great difficulty.

Even then that would not explain the hookup of the master cylinder and booster being hard to push when vacuum is present and the brake lines are off.

That just leaves the master cylinder in my mind. Needs taken apart and inspected for something loose in there may be restricting forward piston movement.

Again ask Hunter if you get a chance before doing anything more. This to me with my limited experience seems a little weird.

whunter · #4 08-09-2011, 02:22 PM

PM your phone number, and I will call asap.

.

proudfoot · #5 08-09-2011, 02:24 PM

Thanks for the replys! As for the brake hoses, I should have done it already, but I am going to get new ones for the front today, and I have already installed new ones on the rear. We'll see if that helps.

In response to Barry's comment "Even then that would not explain the hookup of the master cylinder and booster being hard to push when vacuum is present and the brake lines are off"
I forgot to note that I just have them floating in the engine compartment and am using my hand to push the booster piston in. Not the pedal. When I use the pedal for bleeding w/o the engine on it travels easily to the floor as there is lots of leverage. Just using my palm to depress the brake booster piston alone requires a bit of effort, and the booster and MC hooked together requires a little more effort still. But, with the engine on and vacuum present, it requires less effort to accomplish that task. So the remanufactured booster is doing something. Whether or not that is enough something to help with braking when everything is hooked up, I don't know. As far as I know, there is no way to test the amount of booster pressure I am getting accurately without what I assume would be an expensive machine that would measure the exact pressure required to push the piston a certain distance. All I know is that it is helping some and it is a brand new remanufactured brake booster. Thanks again for the help. I just found some hoses not too far away and am going on a little bike ride.

proudfoot

proudfoot · #6 08-16-2011, 04:52 PM

Hey everyone! Had a great talk with hunter but we didn't quite get it figured out. I have yet to rebleed the brakes and try them out with two new brake lines installed, but I'm not holding my breath for a change in outcome. I'm wondering if my check valve might be bad. When the line is removed (with all other accessory lines capped, although it doesn't change if they aren't) I get 18-20+ hg of vacuum and the needle snaps to attention with lots of pep. I'm wondering if there is a way that the check valve could be failing and still giving me a good reading when I test, but not functioning properly and not allowing the booster to work properly. This is a 25 year old part we are talking about, too. I wouldn't mind replacing it. Thanks!

proudfoot