When I go to hit the brakes in the 240D (hittng the pedal hard, not normal stopping) the pedal pushes back and I lose almost all braking. It has become a major safety issue, I already rear ended an exlorer (lightly, no damage to the ford, just broke my grille insert). The vacuum pump was recently rebuilt (torn diaphrahm, not sure if oil in the system could be causing it?) and I just replaced the front brake pads this weekend. Someone please tell me whats wrong? Any help is greatly appreciated.
Thanks
-Zach

I do know that the vacuum pump/brake booster only makes braking easier. Both of those could fail and you should still be able to brake. Surely nothing like what you described. Beyond that, I don't know. I could hazzard some guesses. Like it might be your master cylinder. You only changed your pads so you shouldn't have needed to bleed your lines. And I have never heard of air in the lines doing what you describe. Have you checked for leaks? How old is the fluid?

Better people will come along to answer your questions more authoritatively.

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Jackson

1984 300d 223k
1994 Jeep Cherokee 2.5L 88k

"She may not look like much, but she's got it where it counts, kid. I've added some special modifications myself." -- Han Solo

"Would it help if I got out and pushed?" -- Princess Leia

You might have air in the system. Did you bleed it properly?

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Youll need to be more specific. The pedal pushing back isnt clear to me. Does it buck back? Does the pedal drop normaly then all of a sudden start to push back against your foot? There is nothing in the brake system that could cause pressure to force the pedal back that i know of so I suspect a mechanical problem with the pedal linkage. However, we really need a much better decription of the problem.

Since the defective brakes have already caused one accident although minor you are right to get serious. I would start with a new master cylinder. Not that much depending where you source it out. Also replace all your brake flex lines if they look old and tired. I do not think they are the cause of your problem but any expenditure is far less than an accident will cost you. A complete flush out of the brake fluid and replacement is in order as moisture in the system might be turning to steam in the front brake cylinders for example and forcing the pedal back. The only other thing that I can think of that could cause this is a defective booster or perhaps the check valve being constantly open or leaky in the vaccum line to it. Disconnect the booster vaccum hose and take a drive in an area where you do not require full braking. If the pedal does not force back at you that may indicate the booster or it's check valve is defective. If it does force back with the vaccum disconected you are most likely boiling the brake fluid. You will not have a lot of brakes with the booster vaccum line disconected so be careful. The first few items I list are just good preventative maintenance on an older car to start with. Let us know what it was as few of our examples if any force the pedal back at you to my knowledge. But again I do not know that much. Find the problem fast because concievably it could get worse..

Its def not boiling fluid. When that happens the pedal goes to the floor. Ask me how I know that and I could spin you a tale of 80mph terror. When water int he fluid boils it turns to gas and since compressing gas doesnt work in our brake systems the pedal just flops down as if the brake line burst. I would lean to the booster and Barrys idea to disconnect it and take a low speed drive is a good one!

What happens when you apply the brakes normally?

Meanwhile try this test for the master cylinder: Sitting at red light put steady and firm pressure on the pedal. If it slowly goes to the floor then MC seals are suspect.

(edit: otherwise i wonder if the rubber ring seal between the master cylinder and brake servo unit is shot.

This happened to my 240D/77 a long time ago. Whatever that it was, it was a vacuum related problem, even after replacing the entire master cylinder and the booster I continued to experiece this problem, off and on. They kept changing brake and vacuum related parts for a while and finally it stopped.
When the problem occurs there is zero brake and the padel is essentially stuck at up position and no amount of force can press it down.
I had vacuum pump failure in the past, the effect on brakes is similar, but in this case these is a tiny amount of brake power left allowing you to stop the car.
It is pretty dangerous, get it fixed asap.

Vahe
240D/77 350K

Sure sounds like a vacuum/booster issue to me. When you step on the pedal the booster assists pushing the pedal down. If the booster stops assisting, it will feel like the pedal is pushing back. Really, it isn't, just that it suddenly got a lot harder to maintain the pedal where it was. This could be caused by a failing diaphram in the booster, a bad valve, or a big vacuum leak. Have you been adding brake fluid? Sometimes the real seals in the MC go bad and leak fluid down into the booster. This causes all kinds of problems up and down the line. I would probably check over the vacuum circuit real careful and if it seemed good, I wouild pull the. If you have been adding brake fluid, don't be supprised if you find some of it in the booster (don't get it on the paint!).

First Off, thank you all for the quick responses.

I took off the vacuum line to the brake booster and tried a few hard stops in my driveway, and I had no problems. It does seem like I am losing assist on hard stops, because if I really muscle the pedal to the floor (pushing with my heel), I can still stop almost normally. I do not know if any of this could be caused by oil getting into the vacuum system when the diaphram in the vacuum pump tore, right before the rebuild? The brake fluid doesn't seem like it is old, its clear, I don't know if thats just how it is even if it is old?
So at this point I'm just trying to narrow things down and kind of establish some plan of attack.
Thanks again

-Zach

Somewhere along the way, I remember being told that the symptoms you describe are due to a bad booster. Something about when the brake pedal is depressed the torn diaphram in the booster moves into a position that allows a vacum leak, thereby making the brake pedal push back. It's pushing back because the booster is losing power assist for some reason.

Hmm, Its seems everything points to the booster I guess. I've priced tham, and now I was just wondering if anyone could tell me how involved it would be to replace the brake booster?
-Zach

You'll have to drain and remove the master cylinder, undo the hard lines going into the master cylinder from the brakes (both front, and the one to the rear) and then undo the four bolts that go through the firewall of the car under up inside the drivers kickpanel area, these hold the booster into the car, once they're loose and you undo the pin that holds the brake pedal lever to the pushrod on the booster, it will come right out. Reverse this process with the installation, taking great care to not get any brake fluid into the new booster (hold master cylinder nice and level when lining it up to bolt it on) and don't forget a gasket between the master cylinder and booster to prevent vacuum leaks.

Once its all bolted in and you've doubled checked everything, proceed to add brake fluid and do a full brake bleed on the car from each wheel, once this is done (easiest is to use a powerbleeder device) then the brakes should be good as new! It sounds like a lot but the brake system is really pretty simple and even for someone not experienced in it should not take more than 6-7 hours total to do if one goes very slowly and carefully. I had to remove and reinstall the whole system on our '83 for the first time last december, and it seemed "scary" at first, but its really not that bad. Others here on the board I am sure will offer other tips and advice as well.

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Did you rule out the rest of the vacuum system? The booster is fairly expensive. Before I went and bought another booster I would first double check that you are getting the correct amount of vacuum to the booster (around 20 to 25 inches I believe), then test the booster itself. The "proper" way would be to follow the FSM. It lists like 6 things that could be causing a problem other than a "bad booster". The quick and dirty way is to press the brake pedal 4 or 5 times with the car off to get rid of any vacuum in the booster, then press down slightly on the pedal as you start the car. If you feel the pedal pressure ease up, like it is getting sucked to the floor a little bit, then that means your brake booster is working...at least marginally.

__________________
Jackson

1984 300d 223k
1994 Jeep Cherokee 2.5L 88k

"She may not look like much, but she's got it where it counts, kid. I've added some special modifications myself." -- Han Solo

"Would it help if I got out and pushed?" -- Princess Leia

Tired old brake fluid is very dirty and not clear. I have always thought some rubber components breaking down a little over time contribute. Sometimes the fluid is almost totally black. I would check the check valve on the vaccum line to the booster. A blow test one way than another would prove it was ok or bad. I might also take a chance on a used booster from an auto wrecker if too expensive new/rebuilt as long as it is guaranteed.