Mysteriously terrible brakes - can we identify a culprit?

We had an Elantra years ago that had the same confounding problem. Each shop I took it to gave me a different BS answer. It wasn't just "normal" for a Hyundai. It wasn't the fact that it needed pads/rotors too. It was a non-ABS car, so couldn't have been that. I too bled the brakes, bench bled and replaced the MC, bled the brakes again, and so on to no avail. One suggestion I received, but never got around to, was to replace the MC again. The thinking was that the replacement may also have been defective. I don't recall the booster coming up as a possibility then, and I don't know how the booster could cause these symptoms, but I agree that it might be worth trying.

One thing I ran into was that when replacing the (also failing) brake light switch, I noticed that the switch itself was acting as the pedal stop. I can only theorize as to why, as it doesn't make much more sense than the booster, but backing the brake light switch out just enough for the MC internals to be the pedal stop instead at least made a significant improvement. It still didn't get the brakes capable of locking up before the pedal hit the floor though, so also not the main problem. We ultimately got rid of it not long after that, before fully resolving the problem.

Went through a similar thing with my Firebird, did a bleed via the scan tool and that fixed it.

Driven5 said:

We had an Elantra years ago that had the same confounding problem. Each shop I took it to gave me a different BS answer. It wasn't just "normal" for a Hyundai. It wasn't the fact that it needed pads/rotors too. It was a non-ABS car, so couldn't have been that. I too bled the brakes, bench bled and replaced the MC, bled the brakes again, and so on to no avail. One suggestion I received, but never got around to, was to replace the MC again. The thinking was that the replacement may also have been defective. I don't recall the booster coming up as a possibility then, and I don't know how the booster could cause these symptoms, but I agree that it might be worth trying.

One thing I ran into was that when replacing the (failing) brake light switch, I noticed that the switch itself was acting as the pedal stop. I can only theorize as to why, but backing the brake light switch out just enough for the MC to be the pedal stop instead at least made a significant improvement, even if it still didn't get the brakes capable of locking all-4 before the pedal hit the floor. We ultimately got rid of it not long after that.

The booster can cause these issues because a lot of them have the reaction disc within them that limits the assist during the travel of the brake pedal.

When the reaction disc is missing, you get full assist and it makes the pedal feel very spongey.  Then, when the brake pads are in contact with the rotors, it is VERY touchy and hard to modulate.

In other words, a bad booster (reaction disc) CAN cause a spongey/ soft pedal.

All the pads thick, rotors decent, visually inspect the calipers for leaks and function.   Any high spots in the brake line routing?  Another thing you can do is break loose fittings one at a time and let some fluid out then retighten, then work back to the mc.  When you loosen the bleeders does fluid flow out?  

Check for anything knocking back the pads, warped rotor, loose wheel bearing.  If the brakes are brand new they might need to be run a bit to get the pedal back, but sounds like either air or worn pads or blown caliper piston.  Usually a leak will pump up, then bleed down.  

The mystery continues. We took out the master cylinder, the ABS module, and the brake booster to try to figure out what the failure point is. Right now, we're having a lot of trouble bench bleeding the master. It's two parts, and the part of the master cylinder that attaches to the reservoir is new. The other half with the bleeder screw is not available as a standalone part. We can't seem to get any fluid out of the bleeder screw at the top of the old cylinder and the piston isn't firming up. Any thoughts?

Right now the hierarchy of likely faults seems to be this: If we can't get it to bench bleed, it's the front master cylinder. If we can, then we'll put it together and take it to a shop to bleed the ABS module. If that doesn't fix it then we'll replace the booster.

Dude, I have never seen that set up before. I will do some research tomorrow. Where does the part that you have the bleeder hose attached to get its fluid?

the clear vinyl tube? we have it going to a bottle of fluid.

Edit: Oh wait, I see what you mean. Let me see if I have a pic

The two disconnected brake hoses in the foreground attach to the new part of the master cylinder (removed in this picture).

So the one on the top goes to the ABS module and the one on the side goes to the other half of the master cylinder.

I'm having trouble understanding your bench bleeding setup. 

I've always bench bled by filling the reservoir and installing fittings with tubing in the outlets with the tubing routed to the reservoir, then cycled the master cylinder. Fluid is pumped out the brake line port, through the tubing and back to the reservoir, this is continued until no more bubbles are seen. 

If you are trying to pressurize the fluid while bench bleeding I don't think you'll get the air out, or not easily. 

In reply to wvumtnbkr :

That's kind of my point. The type of soft pedal symptoms a bad booster creates are different than the type of soft pedal symptoms in the OP. 

In reply to No Time :

My master cylinder came with white plugs like that to bench bleed. They're inserted as fluid comes out of the ports, they don't seal completely (at least not on mine) and you keep pumping until bubbles stop coming up the reservoir. It was messy.

In reply to j_tso :

Correct, we let the reservoir fill the MC, installed the plugs, and pumped the air out the reservoir.

Ok, we took the whole thing apart, put it all back together, and bled it on the bench with the brake line back in place to transfer fluid into the upper chamber. We have confirmed that brake fluid shoots out of every port when pumping the pedal, except for one very notable exception - when you pump the cylinder the bleeder screw at the top of the upper chamber actually is in vacuum. The chamber is completely full - it drips when you aren't pumping the pedal - but when you pump it it actually pulls a small vacuum instead of pushing fluid out, even if you remove the screw completely.

If you tighten the screw and close it, no matter how many times you pump the pedal it does not build pressure. Nothing looked damaged inside, but I can't figure out how this is supposed to work. It seems like it just recirculates fluid inside the MC instead of trying to push it out. It doesn't make sense, as far as I can tell everything should work properly.

dude that's so berkeleying weird.

i haven't researched it yet.  lemme look into it before quitting time today.

OK, so it looks like that van must have stability control.  the part that goes between the MC and the Booster is how the stability control system gets fluid from the MC to do a pressure build when the brake pedal is not being applied.  it's a complicated way to do it, but probably saved them from having to license someone else's patented design.  so i would never expect that part of the system to have pressure in it, hence the no fluid squirting out when you bleed it.

and i can add this to my personal TIL list

In reply to AngryCorvair (Forum Supporter) :

Good to know, that's very odd.

We bench bled the MC properly, put the new booster in, hooked everything back up, and did a bleed with a scan tool to fully bleed the ABS system. When the engine is off, the pedal feels great. But once you turn the car on the pedal will push straight to the floor with very little resistance, and it's slow to come up. If we pinch the vacuum line then it feels a lot normal. Could something cause an excess of vacuum?

AMiataCalledSteve said:

Could something cause an excess of vacuum?

See wvumtnbkr's comments on the reaction disc, but my (limited) understanding is that the soft pedal feel from excessive vacuum in the booster also shouldn't significantly lower the engagement point nor reduce the effectiveness of the brakes themselves.