Brake problem

[Layne]

The car is finally able to be driven! But there's a problem with the brakes. They're hard to press, and I can't lock them up, no matter how hard I press the pedal. Normal braking takes force, but works fine. There's no feeling that the car won't stop.

The booster is a smaller unit (I was told it goes to a 2002, but it doesn't look like a factory one so who knows) and it doesn't feel like there's a ton of boost, but I would think you should still be able to lock up the brakes if you press hard enough. You can on any other car I've tried. The master cylinder and calipers are all OEM spec and all rebuilt by me.

The piping had already been messed with before I worked on the car, but I assumed it was correct, the left two ports go to the left front caliper, and the bottom two off-center ports go to the right front caliper, and the bottom center port goes to the rear (through the pressure reducing valve). I don't recall which caliper piston the front and rear MC ports go to, but it's the same on both sides. I assume it would not matter if this were crossed up...?

I have two theories in mind... 1) The stock booster is HUGE, maybe the brakes are inherently extremely hard and the stock booster delivers much more boost than other cars. 2) Perhaps only one circuit of the master cylinder is working. Does anyone know what that feels like? All bleeder ports will pump fluid out, but maybe one circuit isn't making much pressure due to a mishap rebuilding the master cylinder. Seems unlikely. Would be nice if I had a pressure gauge. Any other ideas?

Here's the setup. Again, it's all stock except for the booster itself and the shortened bracket.

And here's the car, now in near-functional condition:

 

[HB Chris]

I've never seen that booster, Tii version is longer.

 

[sfdon]

You didn't use dot 5 right?

One test- open bleed screw while car is running
Step on brakes
Fluid should come squirting out steady strong stream
If so calipers or fluid dot 5
If not it's booster or master?
Pump brake 15 times- with foot on brake start engine. Pedal goes down?

 

[Layne]

Pump brake 15 times- with foot on brake start engine. Pedal goes down?

Yes, it does a little now that I found and fixed a vacuum leak on the input fitting. Not as much as most cars. Again, you should be able to lock the wheels with no boost at all.

No I didn't use DOT 5. I used BMW brand DOT 4.

 

[aearch]

IS THE VACUUM HOOKED UP TO THE BOOSTER
I DONT SEE
NO VACUUUM NO BOOST
FLUID MAKES NO DIFFERECE IN STREET CARS ONLY RACING

 

[Ohmess]

Are you sure your vacuum check valve is properly installed and working?

 

[sfdon]

Use your handheld vacuum pump/gauge to measure vacuum to booster while running and pushing down on pedal

 

[Layne]

Are you sure your vacuum check valve is properly installed and working?

Yes, my first thought was that I installed it backwards.

 

[Layne]

So today I learned the booster is just a universal ebay type item, and it is a single diaphragm. I just assumed it was a double diaphragm to make up for the much smaller size without really looking into it. The previous mechanic picked this one out. So far every single move he made has bitten me in the butt, I should have known to throw out this booster and start fresh (although the booster install was one of the first things I did. Easy with the engine out). Compared to the original booster, this one has half the area, thus half the boosting force. A double diaphragm unit of the same size should be almost identical to the original. They make a similar universal double diaphragm unit, but of course it's longer, so all the pipes will need to be modified.

In light of all that, I suppose I'm wrong in my assumption that I should be able to lock up the brakes with no boost and there probably is no hydraulic problem. If someone wants to disconnect their booster and give it a try I would much appreciate it, but I doubt anyone will.

I found this information from speedway motors.

Here's what the two versions of 7 inch booster look like. Single:

Double:

 

[Ohmess]

Dude - I have a stock booster laying around if you need one.

 

[Layne]

Dude - I have a stock booster laying around if you need one.

Won't fit, motronic engine.

 

[Tony.dreamer]

SFdon is giving you all the right starting points.
I assume you have CS3.0 car with all 4 wheel disc. I see from the photos you have new metal lines and I assume your rubber brake hoses are new as well...
With my brake problems I start with simple items checked first. That will save you tons of time
- it is essential to have steady stream at the bleeder screws. Check them one by one. You should have steady stream of fluid with each pumping of the brake pedal and holding the pedal.When bleeder screw is open there is no pressure in the system , and if there is no obstacle (broken debris of brake hoses, clugged lines , pedal action should directly correlate to squarting of fluid.
2- if item 1 checks out fine, I would then go to each calipers and with a large screw driver I would try to move the caliper pistons back (and forth by having some one apply the brake) by wedging the screw driver in between brake pads and discs all the way in and see how freely they move. I am not sure if you honed the inside wall of the caliper cylinders or re-sleeved them but I have had many occasions where calipers are not moving as freely as they should causing the same issues you are having. ....On older Mercedes especially ... That has happened to me EVEN AFTER THEY HAVE BEEN REBUILT mainly because they were not rebuilt properly.
3- if you are happy with item 1 and 2 then you should check the vacuum first regardless what size or type booster you get . The amount of vacuum you have available is crucial before any booster can use that and translate it to extra braking power. You can have a ...super duper double mega tron ..booster and if you don't have enough vacume it will not do you any good. With your new motorics instaled I am not sure as I am learning more about E9 engine options but where do you get your vacume from and how much vacume is available, and how fast it build up (recovers) is crucial . After you verify proper amount of Vaucme no less than 13-14 pounds preferably more then you should press the pedal and see how much it drops and how soon it recovers after you let go of the pedal....
4- if all checks out fine then you should check or replace your. "Check valve "...

Also you should make sure that during your brake installation your disks and pads were really clean and free of any oil causing your issues. You will be surprised how much that can contribute to your problems . Before I put my pad on I use brake cleaner and completely clean the disks and pads and then lightly sand the top layer of the pads.......

Majority of what I said SFdon pointed out , I only expand on it...

Best wishes

Tony P.

 

[Gransin]

The thing that makes me suspect something else than the booster is this:

I can't lock them up, no matter how hard I press the pedal.

A non working or broken booster shouldn't affect your possibility to lock the brakes if you press hard, or like in this case, almost stand on the pedal.
It is of course possible that the booster is much less effective than the original one, but I suspect you also have some other problem. Air, bad M/C, calipers or oil on pads/discs as has been said before.

 

[bluecoupe30!]

yeah, I'm thinking master cylinder too.

 

[sfdon]

"Won't fit, motronic engine."
You mean the motronic intake manifold?

Answer to your question- I'm 6'2", 225 lbs with size 13 1/2 shoes.
I just pulled the vacuum line to the booster and tried to lock up the brakes 5 times.
Not even close to locking up anything.

 

[Gransin]

Answer to your question- I'm 6'2", 225 lbs with size 13 1/2 shoes.
I just pulled the vacuum line to the booster and tried to lock up the brakes 5 times.
Not even close to locking up anything.

Interesting!
I've always been under the impression that the booster mostly helps when a mediocre amount of force is put on the pedal, and not as much during full panic braking and so on.
I've seen people ditch the booster completely in builds where it simply didn't have room, and to my knowledge kept the standard M/C. (other cars than e9)

But I can't say I've ever experimented with it myself, and not on these cars, so I'm obviously wrong

 

[Ohmess]

Another question is whether you have recently replaced all your rubber hoses (stainless is recommended as they are basically the same price as new rubber). If not, the hoses sometimes corrode from the inside, releasing little rubber chunks into the system which gum things up.

Yet another thing to think about is rust on the pistons, particularly where they extend outward from the caliper. Did you find any of this, and if so, did you address it when you rebuilt your calipers?

 

[Layne]

"Won't fit, motronic engine."
You mean the motronic intake manifold?

Answer to your question- I'm 6'2", 225 lbs with size 13 1/2 shoes.
I just pulled the vacuum line to the booster and tried to lock up the brakes 5 times.
Not even close to locking up anything.

Awesome! I really didn't think anyone would do it. I appreciate that. I've got a new booster on order, as it is clearly "A" problem, even if it weren't "THE" problem, but I'm now convinced it is.

 

[Layne]

I've seen people ditch the booster completely in builds where it simply didn't have room, and to my knowledge kept the standard M/C. (other cars than e9)

In all cases that I know of, people use a smaller bore MC when they delete the booster. And all cars that I know of that came with or without a booster, the MC is smaller for the non-boost application. Not all stock systems are equal of course, and the E9 has massive brake piston area compared to most cars.

 

[Layne]

Another question is whether you have recently replaced all your rubber hoses (stainless is recommended as they are basically the same price as new rubber). If not, the hoses sometimes corrode from the inside, releasing little rubber chunks into the system which gum things up.

Yet another thing to think about is rust on the pistons, particularly where they extend outward from the caliper. Did you find any of this, and if so, did you address it when you rebuilt your calipers?

All hoses are new braided SS. Didn't see any meaningful rust on the pistons, and anything I did see was cleaned off.