I’m having problems getting pressure to my brakes (spongy pedal) after bleeding them numerous times, already replaced the 3rd gen power booster and the LS1 master. I’m currently doing an ls1/t56 swap so I decided to run the LS1 master since it looks a lot cleaner. Here the specs on my car, 1983 Z28 w/ LS1/T56 running:

LS1 Master
3rd gen Power Booster
3rd gen Proportion Valve
3rd gen front brakes w/Hawk pads
Moser 12 bolts w/ 4th Gen LS1 Rear Brakes
Stock hard lines to front brakes and rear distribution block.
Steel brakes lines to 4th gen LS1 Calipers

My brakes worked great before doing the swap w/ the 12 bolt and the 4th gen rear brakes. I’m thinking about picking up a Z06 brake kit from ebmiller but I want to make sure my current brake system is operating correctly before upgrading.

Here’s what I changed during the swap:

LS1 master
3rd Gen Prop. Valve from an 87 GTA
Steel brakes lines to 4th gen LS1 Calipers (I was running the 3rd gen lines but they were all over the place so I decided to clean them up by using brake lines from Autozone.)

Any help or input is appreciated.

 

Just a sanity check here, but did you bench bleed the master cylinder prior to installation?

Bench Bleeding Article

 

Thannks thewolf and yes, that's exactly what I did to the master cylinder. Then I even did the gravity bleed went out and bought Speed Bleeders for the front (ordering some for the rear also) but no luck.

I searched and according to other members, the LS1 master and 3rd gen power booster are compatible. Is the power booster pin the same length on the 3rd and 4h gen power booster?

 

I dunno what is causing your issues, but I just put a LS1 MC and 1LE prop valve in my 91 RS last sunday. All we (stepdad and myself) did was unbolt the MC and prop valve, then re-installed the LS1 MC and 1LE prop valve.

 

It sounds like air in the lines to me. I have a 4th gen MC waiting to go in, so I can't say for sure that the booster rod stroke and length are correct, but it seemed to me that back when I bought that, several people had that swap working with no issues. You just got done changing a lot of brake hydraulics. I would make darn sure the brake system was bled thoroughly. Not trying to say you aren't careful or anything.

FWIW, I have always used the helper-in-the-driver-seat method of bleeding brakes.

Can you pump the pedal a few times with the car off and get a firm pedal feel? If not, I bet you have some bubbles in there.

 

If the car is off, the pedal gets hard after a few pumps. It's just when i turn it on, that the pedal feels soft and hits the floor when i pump it.

I haven't changed the check valve so I'm going to try that, maybe it's a vacuum related issues?

 

Z28fast1

Did you ever resolve this issue?

 

Lost track of this thread but yes I actually purchased a Motive pump bleeder that hooks up to the master cylinder. The pedal feels a lot better but not perfect yet. Maybe i'm trying to compare pedal feels to newer vehicles since i had't drove the car in forever?

 

I have the exact same problem, spongy brakes with LS1 master cylinder, stock rear disc prop valve, and LS1 rear brakes. When I get done driving all 4 discs are almost the same temp, but on the chassis dyno rollers the rears could hardly hold the rear tires from spinning at idle. And I can't lock up the fronts, not even on sand hardly. It's like hitting the pedal at 50% foot pressure has the same effects as both feet at 100% pressure. I have also bench bled the m/c. I'm gonna gravity bleed all 4 next, 'cause I gotta get this stuff working.