Soft brake pedal

So this goes along with my clutch post. Car is up and running (for the first time) and currently the pedal goes almost to the floor before the brakes catch. I pressure bled the entire system and everything seemed to go as planned. I've been told perhaps I need to bench bleed the master cylinder, but wouldn't the pressure bleeding take care of that?

I'm running the BMW hydroboost system with lexus ls430's up front, turbo brakes in the rear, and steel braided lines, if that matters.

Again, any help/suggestions much appreciated!

JP

I did exactly the same thing with the brakes. I bled them with my power bleeder without bleeding the master and had no brakes. Bled the master and all was well. I'm guessing the air in the passages for the bleeder screws just doesn't come out of there without cracking them open.

Edit: Just realized you said you have the BMW hydroboost and I don't remember if those masters have bleeder screws on them. I'd still try bleeding the master, though.

I find a buddy bleed is necessary with a completely empty master. The bench bleed or a buddy bleed followed by a pressure bleed usually does the trick.

But bleeding probably isn't your clutch issue. I agree that sounds like it needs a shim.

Rich

Same issue with a similar setup (power assist/ls430s front turbos rear/stainless lines)

I've bled and bled- pedal method, pressure method, pedal+pump method. Never an improvement. Even went to a bigger master (bigger than the stock ls430 one) which did improve it but its still not good. Replaced my wheel bearings and everything, even with the car off the pedal sinks a few inches before catching.

Sure am sick of having to "pre-pump" my brake pedal to get it hard before I get to a stop sign.

You have to bleed the master. Thread in the plastic nipple adapters to where your brake lines go and some clear hose on to those and you wont have to take it out. This is a must. Years of brake work.

I can concur. I finally bench bled the master and problem solved.

So turns out problem not solved... At first, bench bleeding the master cylinder seemed to do the trick, but over time the pedal has just gotten more spongey. The brake fluid resevoir is full, no leaks in the brake lines or at the calipers. Hydroboost is leaking a tiny bit of ATF from the steering rack, but not enough to cause the amount of sponge I'm getting, so I'm kinda running out of ideas. My only thought is perhaps air is somehow getting into the brake lines. Perhaps where they connect by the battery?

Currently the pedal goes alomst to the floor before really catching.

Sounds to me like there is still air in the lines. Do it old school. 2 people and pump amd crack the bleeder. Once all the air is gone you should have a firm pedal. Keep us posted. Happy New Year.

Well, I bled the brakes again today and pedal is back to feeling nice and firm. So at least now I know the issue is with air in the line, and not something else. The question is, will it stay air free? In theory it should be fine; you bleed brakes and it's done. But I have a sneaking suspicion air is working it's way into the lines somehow. I triple checked that there were no leaks today at any of the calipers, master, line connections, etc.. Is it possible air can be getting in the lines even if there are no leaks?

Thanks everyone, this has been a big help and I will figure it out, eventually!

its not possible for air to get in if fluid isnt getting out. you brake system pressure can be 1000-1500psi, if that sort of pressure isnt getting out of the lines, air isnt going to get in.

ive run into cases were fluid will get trapped in the master cylinder and you can bleed it for hours and not get a good pedal feel, or get a good feel initially but overnight you lose the pedal. ive found that sometimes you need to unbolt the master cylinder and and tip it in a way that might allow trapped air to get out.

maybe post a picture of your master cylinder in the car, might be helpful.

Sharkey, thanks for the reply. I think you've nailed it. I bet that's what is happening. Oddly enough, when I first installed the system, I bled the brakes twice with the "overnight" loss of pedal feel both time. When I bench bleed the master I thought it was fixed, but over time it softened up again. However, this last time I bled the brakes I did indeed get a few large bubbles out of the rear passenger side caliper. I'm hoping this may be the last of it. So far, so good. Attached is a pic, of my engine bay, I'll take a better one tomorrow.

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Another remote possibility, air is getting pulled in past the piston seals. I have had it happen on my 3/4 ton Suburban with no liquid leaks whatsoever. There are various reasons this can happen, for me it would occur after a big temperature drop in the winter months where the fluid would contract and draw air in past a piston seal. Air would be present in the LF caliper only, sometimes a day after bleeding; these calipers were original and in otherwise perfect working order. Replacing the caliper with a new OE unit fixed my issue. IIRC the C5 vette suffers from a similar problem, caused by vibration though.