Where's my brake pressure gone?
 

Have just installed new rear brake-pad's/rotor's on my `87 300E. Before replacing the rear wheel's (as I had great access to the bleeder's), I decided to bleed the line's. It appears the brake-pressure I had prior to the install has "vanished". Not only do I now have 1/2 the pedal-pressure I had prior to jacking the car up, I am getting nothing out of the bleeder's other than the odd "spurt". BTW, the rear-caliper's were supported (not left to hang) the entire time and the fluid-level in the master-cylinder is still basically full. Any idea's as what might have happened? :nix:

A couple of thoughts come to mind. Assuming that you did not do anything out of line in the bleeding process, you possibly let the master cylinder reservoir go dry. You possibly have a air in the master cylinder. Although I have not seen it in a long time, it is possible that some dirt and grit at the bottom of the reservoir as you are removing fluid during the bleeding process. Also I wouldn't spend any extra time supporting the calipers. I just let them hang. One brake test that is in the textbook says to pinch off hoses with vice grips. They are pretty tough little Devils.

Thank's for your thought's. As it has gone, the level in the master-cylinder never dropped at all. The caliper's were able to rest (up-right) on the floor as the car sit's somewhat low. It appear's, as odd as odd's can be, that the master-cylinder itself has suddenly failed....no warning at all!? Any other idea's?

If you think the master cylinder may have failed,it is for the reason that I mentioned above. As the fluid level in the master cylinder drops during the bleeding process, grit and grime from the bottom of the reservoir enters the master cylinder and gets between the rubber cup and the bore. This causes fluid to bypass in the cylinder and not apply pressure to the calipers. This could be your problem. Most things don't come apart these days, because they're meant to be thrown away, but you may be able to disassemble your master cylinder. You can remove it and see if there is a removable snap ring at the end that allows you take the guts out, clean everything, and reassemble. Good luck.

Thank you again for your in-put. If only the fluid-level had dropped. It never did. We were literally 1-pump into the bleeding-process and that-was-that as nothing other than an occasional "spurt" ever came out. BTW, the brake-pedal still has some resistance....doesn't just go straight to the floor!?

At this point I don't know what to tell you without being there myself. What type of bleeder system did you use? There have been all sorts of threads on sophisticated bleeding equipment.For 30 years I have faithfully used the system the German's taught me when I apprenticed at Mercedes. I hook up a hose and bucket just to avoid the mass, open the bleed screw, and let that thing called gravity do the work for me. Larger shops buy expensive pneumatic bleeding systems, because it is supposed to be faster and save shop time. That is not necessary if you plan your work. If you're going to bleed brakes, hook up the drain bucket, open the bleed screw, and do other things while the system is bleeding. You can check tire pressure, condition of driveline, remove oil drain plug, etc. Let me know what bleeding system you used and was the car level at the time.

As an afterthought when bleeding brakes is a good idea to use a clear hose to watch for solid fluid with no bubbles. A good tip to follow is to only crack the bleed screw and don't open it several turns. The reason for this is you create a Venturi effect around the screw threads and suck in false air giving you the illusion of air in the brake system.

Thank's yet again hineywineries. We were using the "two-man" method as I had lost my pressure-bleeder to theft.

Other than the economic loss that may be the best thing that could've happened.:DAt this point I suggest that you remove the reservoir, clean it thoroughly, reinstall and put in clean new fluid. Save your help her out for beer, because you don't need him or her. Using a piece of clear tubing gravity bleed each caliper one at a time. Also you did not indicate where in the country you live unless I missed it. If you live in the rust belt there is a possibility that some of your bleed screws may be rusted or clogged. You may want to remove each bleed screw and be sure fluid can pass through them. I have seen this before also.

Hi again. I forgot to tell you I am in Oregon (that's why you couldn't find it). I have only been opening the bleeder's about a 1/4-turn each. The bleeder's look almost like new (as in no-rust/corrosion/crud, etc.). I am going to try your "gravity" method. Do I need to have the pedal depressed (as in an object placed against the pedal to hold it down during the process) to get the gravity to work with me?

Good question but you don't need anything on the brake pedal. If you put something on the pedal, it wouldn't be gravity anymore. It would be pressure. Just make sure the reservoir doesn't go empty and do one wheel at a time. It is best if the car is level.

The car is basically level. I go give it a go right now.

Well, to date, we have attempted "gravity-filling", replacing the master-cylinder and bleeding with the "Mighty-Vac" bleeder (just delivered to me yesterday) and still no brake-fluid at the bleeder!? Time for a bloody drink!

Which breaks are you trying to bleed at the moment? Are you at the front brakes or the rear?To rephrase that, where are you not getting fluid?

The rear's.