Mysteriously terrible brakes - can we identify a culprit?

My brother has a 2006 Toyota Sienna with atrocious brakes. They are crazy spongy/soft and don't engage until very low in the pedal. You can pump them up to get more resistance, but they will go back to their limp state again the next time you brake. It feels like if you pump the brakes and hold them, the pedal goes down, but almost imperceptibly slowly. He took it to a shop just the other week who told him it was the master cylinder and replaced it for him. Unfortunately, the brakes were still terrible. We bled every wheel and the master cylinder again. We couldn't find any external leaks. The brakes are just as bad as ever. He took it to another shop today who said it appeared to be a master cylinder problem, but given it was just replaced, is there another culprit that could be causing such a persistently terrible feeling?

If you have access to a scan tool with ABS functions you should be able to cycle the ABS module and then try bleeding again.
 

If you don't have a scan tool with the capability, find an empty dirt parking lot and make the ABS engaged through some hard stops to try and force air out of the solenoids/valves, then bleed again.

Also check all the slides and calipers at all 4 wheels to make sure nothing is frozen. I'm not sure about the 06 Sienna, but most cars now use the ABS for proportioning instead of a  mechanical valve, and sticking calipers/slides or pads in the anchor brackets can cause funny behavior. Although I don't necessarily think this would be the cause (I think it might be air in the ABS), it wouldn't hurt to check before throwing any parts at it. 

Thanks, air in the ABS definitely sounds like it could be a possibility. I don't have a full scan tool, but I know some good gravel roads nearby...

Do the calipers bleed? 

Check the soft hoses. Sounds like one dry rotted and is slowly losing pressure and letting air in.

I agree with checking/replacing the hoses.....they're probably original to the car......

Drums or discs in the rear? I just discovered my persistent and identical brake problem was caused by the drum brake self adjusters failing, so every time you hit the brakes, they had a massive take-up before the shoes hit the drum. If you pump the brake once and then hit it again, it felt great. Eventually found the issue when I accidently didn't release the parking brake all the way, which actuates the shoes from the opposite end, and it took me a second to figure out why my brakes suddenly felt better than ever.

edit - I don't have the sinking pedal problem, though. That can pretty much only be a bad seal somewhere. Even air in the lines usually doesn't sink over time, it is just spongy.

In reply to MiniDave :

Wouldn't bad hoses have a visible leak somewhere?

In reply to lnlds :

They do, we bled them yesterday.

Weird suggestion....

There is a little piece of material in the booster that can be dislodged.  It is called a reaction disc.

If it gets dislodged, it will act exactly like what you are describing with the 1 big exception being the pedal shouldn't drop.

If the pedal is indeed dropping, fluid is either getting past the master cylinder piston, something is leaking somewhere downstream, or something is bending / flexing somewhere downstream (or firewall).

just chiming in to say that all the options have been covered, except for bad replacement MC out of the box.  it happens more than it should.

AngryCorvair (Forum Supporter) said:

just chiming in to say that all the options have been covered, except for bad replacement MC out of the box.  it happens more than it should.

We've been educated by the best!

These are all great suggestions, thanks everyone!

We put the van up on jack stands and I can't see anything that looks like a leak. We tried running the car down the gravel driveway and engaging the ABS a bunch, but I haven't tried bleeding any of the calipers after we did that yet. It's hard to know if the pedal holds steady or is dropping because when I press on it really firmly it goes straight to the firewall (with some resistance). My guess is defective MC, but we'll try replacing that tonight and see how it goes I guess.

One other thing: When I took the wheels off, all of the bleeder screw caps were missing. I know I put them on last night, I don't know how they all disappeared.

Once had a similar problem like this on a bugeye Impreza where the leak was something internal to the ABS system (likely the reservoir), replacing the ABS unit fixed it. I had a thread on here about it but can't find it now...

Edit: Found it: https://grassrootsmotorsports.com/forum/grm/please-help-with-terrible-brake-enigma/142184/page1/

In reply to wvumtnbkr :

Is there any way to test for this reaction disk?

In reply to GameboyRMH :

That's one I wouldn't have thought of, hopefully that's not it but I'll read your link, thanks

AMiataCalledSteve said:

In reply to wvumtnbkr :

Is there any way to test for this reaction disk?

Not that I am aware of unfortunately.   Do the brakes grab really hard at the end of travel?  

Like nothing...  nothing.. .  Panic stop?

In reply to wvumtnbkr :

Not exactly, more like nothing... nothing... soft braking 80% of the way down, then right at the veeeery bottom you get hard enough brakes to make a decent stop.

Hey,

We had a similar problem on a '96 Blazer.

The cause was a leaking/failed brake booster.

Rog

Welp, we replaced the master cylinder and bled the bejeezus out of it and the problem persists. I'm gonna have to go out on a limb here and guess that the master cylinder wasn't the problem...

The pedal pretty much goes straight to the floor. It sounds like y'all think the brake booster is the next culprit?

It could also be bad calipers or wheel cylinders.

Weirdly. Yes.  That was the issue on my rx7 racecar.

Boosters shouldn't make the brakes softer....  said everyone.  

Changed the booster, problem disappeared.

My brother is going to take off and inspect each caliper to check for any signs of poor functioning before we do anything else. There was on caliper that had a little rust come out when we first bled it, so he'll inspect that brake line too.

One caliper had a sticking pin, but that didn't solve the problem. My brother clamped the vacuum hose going to the brake booster and it firmed up significantly, so we're thinking the booster might be the problem after all. I can't really think of anything else it might be...

I also recently had this problem with a bad reman booster. Unhooking the vacuum line also stiffened the pedal immediately.