Soft brake pedal when car is on.

[memsiej]

Y'all have been super helpful in the past. A few months ago, I ended up replacing the MC and brake booster on my 84 GW only to find the problem was a rust-welded brake pedal pivot. No matter. Just now got around to finishing it up; bleeding the MC and wheels to get the brakes back online. After bleeding the MC and all 4 wheels (from farthest to closest), the brake pedal was nice and stiff but as soon as I turned the car fully on the brake pedal became very soft.

I've bled the wheels multiple times now. I've tested and bled the MC many times, and even tested, bled and reinstalled the old MC. Still getting a soft pedal when the car is on. Had wife pump the brakes with car on and I looked everywhere for signs of leaks but found nothing.

Of course, without brakes, I can't drive it to a shop. Anything else I can try?

[SJTD]

Does the pedal stop moving before the floor? Does it stop ok?

Sounds like maybe the old booster wasn't working properly and you're now getting more assist?

[memsiej]

Pedal stops before floor when the car is off, but when the car is on it goes all the way to the floor.

[tgreese]

Did you buy and install a new master cylinder? How long has it been? Did you bench bleed the master cylinder?

If the pedal gets soft with booster assist, then the extra strength of the boost is pushing the fluid around the piston in the master cylinder. The booster makes you stronger.

If everything is put together right, the link between the pedal and master cylinder is solid; nothing squishy beyond a fraction of an inch movement to operate the air valve in the booster.

[memsiej]

Yes- bought, bled, and installed a new MC about a month ago. Also to be sure I tested, bench bled, and tried the old MC again.

Still pedal goes to the floor only when the engine is running. Can see bubbling in the MC when I push on the brake pedal, but cannot find a leak anywhere.

[will e]

Bubbling means air in the line (as you know). My pedal also gets much squishier when the engine is running but it brakes fine and the pedal does NOT go all the way to the floor.

[tgreese]

Last time I had a persistent problem with my brakes, I bought a power bleeder. This helped a lot with bleeding. Turns out the caliper was frozen on the left front. A new (reman) caliper fixed it.

Following Will, that air has to come from somewhere. If there's no air, you can see the fluid fountain but not bubble. Air has to be coming from the lines or is trapped in the cylinders. There could be a leak in the lines or hoses, I'd guess. More likely you have a big bubble somewhere that's migrating bit by bit back to the master cylinder when you cycle the pedal.

[SJTD]

Yeah bubbles in the master means air wasn't removed. Nothing to do with engine on or off except that with power you can compress air in the system easier.

Did you bench bleed the new master?

[memsiej]

Yea I bench bled the new master half a dozen times now, as well as all the wheels. Maybe there is truly still air in there?

[Werndogz]

I've been having the exact same problem with my 85 Toyota, it brakes ok but the pedal is just super squishy until it gets about an inch from the floor, interested to find out what's wrong with yours, maybe I have the same problem.

Sent from my Pixel 6 using Tapatalk

[Stuka]

Yea I bench bled the new master half a dozen times now, as well as all the wheels. Maybe there is truly still air in there?

Did you hold the button on the proportioning valve in?

[sierrablue]

If I had to guess, with all of your swapping the master cylinders around, you have LOTS of bubbles in there. Probably since it was the master cylinder, they're towards the top. You might try "gravity bleeding" if you haven't already--put a drain pan under the wheel farthest from the cylinder on the fronts and rears, and open the valves a quarter turn or so. Then just let it drip for awhile, keeping an eye on the level in the cylinder (it will speed up as you let it keep going. Then I'd recommend using a vacuum pump of some sort, with someone pumping the brakes.

Just a note, don't pump the brakes while the valve is open (well don't lift off anyway) unless you have speed bleeders, as this will suck air back into the system. If you have speed bleeders, you'll have to take them all the way out to do the gravity bleeding.

You might also check that the rears are adjusted properly; if the pads are too far from the rotor, it could be that it feels fine w/o boost but with the boost it's trying to push the pads out a long ways.

Just my .02, not trying to say you don't know what you're doing. My brakes aren't done being bled rn, but they're plenty solid enough to stop it. Just when you think you're done there comes another string of bubbles, and magically the pedal starts to firm up again.

[tgreese]

I would add - if you install new linings on the rear, you must adjust the brakes initially. The pros use a special tool to do this.
https://www.amazon.com/GearWrench-3377- ... B0002SQUJ0
You then follow the instructions in the TSM to finish the adjustment using the self-adjusters. The self-adjusters must be assembled properly; they are different left and right. Again, the TSM will show you a picture that you must follow.

You can use a brake spoon and screwdriver to make the initial adjustment, but the special tool is __much__ easier, and success is much more likely.

The linings are held away from the drums by strong springs. These could feel solid with the engine off. Start the engine and the linings suddenly move easily.

Realize that you have to 1) get all the air out of the system and 2) make an initial adjustment so the movement of the pads and linings is small compared to the (not large) fluid volume the master cylinder moves. Liquids are incompressible, and gases are compressible. You need 100% liquid (fluid) in the system - no air!

Drum brakes are easy, but they are mysterious to most modern mechanics, shadetree or otherwise. Brake drums wear faster and have more moving parts than disks. The TSM will help you here. If you are flailing, suggest you replace everything for the rear brakes that's available - linings, drums, springs, self-adjusters, pawl, pins, etc. Don't cheap out on brake parts.

[sierrablue]

Just adding to what tgrease said--everything I've been told is to adjust the pads out until they lightly rub on the drums. If you can hear the shoes moving when you step on the brakes, they're too loose, moving too far.