Soft brake pedal when running

[Blazerowner]

Found this guy. I don’t know if I’m having the same issue as it’s all new but who knows. I don’t see any air while it’s bleeding either.
https://m.youtube.com/watch?v=tYYMoZGEzCc

[geezer#99]

Another thought to add to the problem.
How much vacuum does your motor produce at idle?

[Blazerowner]

That would be a hard one to answer, but it’s a stock ls3 with a stock intake. I’m using the rear brake nipple. Seems to pull good vacuum when I pull the hose off but I don’t have a gauge to know the exact amount.

[geezer#99]

It’s just one possibility, likely not your problem.
Low vacuum causes a hard pedal.
I shouldn’t have mentioned it.
I did some searching and two things that popped up where the pin adjustment and the drum brake adjustment.
I’d start with the drums since it’s the easiest.

[Blazerowner]

No worries I take all suggestions with open arms. I tried adjusting the drums but no change. I’m going to isolate the fronts with a plug on the rear circuit and see if that firms up the pedal. If it does I’ll know it’s something with the rears.

[LS short box]

I've had good luck with the Motive pressure bleeder. Also the Mity Vac vacuum bleeder.
What size cam do you have in the LS? Plenty of vacuum?
Maybe not a big deal but those small diameter vacuum boosters can be a problem as they don't provide much assist.

[PbFut]

It seems you have ruled out anything at the wheel, and no line leaks so we are up to the MC or maybe the booster. You description reminds me of a problem I had 50 yrs ago in high school on a non-power brake Camaro. I would hit the brakes, they worked but slowly the pedal would creep to the floor at a red light. I would lift and quickly reapply. The method got me by till I was talking with my neighbor. He had me go buy a master cylinder rebuild kit. Showed me how to use a brake honing tool and an hour later I had perfectly working brakes that lasted years. Long story to get to maybe the MC is blowing by the seals when higher pressure of the booster is applied.

[Blazerowner]

I’m hoping it’s not the master, but I’m not ruling it out. I’m waiting for a plug to fit the rear circuit so I can test the front to see if I get pedal. Luckily it’s winter and I’m not needing to drive anytime soon.

[Steeveedee]

A bad (bypassing) master shows up with the following test- Stand on the pedal, hard. Slowly let off the force, and if it is bypassing, the pedal will sink slowly to the floor when not much force is applied.

[Blazerowner]

I finally got out today to do some testing. I’ve been fighting a bad cold for the last few days. So what I did was capped the rear circuit at the proportion valve and started it up so I had vacuum. I have a nice hard pedal with the rear circuit capped so I know it’s something to do with the drums. I’m wondering if it is one of the cylinders. I may try to bleed it one more time with the vacuum bleeder I just bought and see if it’s air. If that doesn’t work I’m going to buy a couple of new wheel cylinders and swap them out. The guy in the video above states they can suck air and you all know the quality of newer parts now days. Hopefully I can get this done this coming weekend when I feel better.

[PbFut]

I used a vacuum bleeder on my 72. One side on rear blead just fine. The other barely got any fluid out. Removing the nipple revealed a bunch of dirt and rust in the nipple bore. Required a small drill bit and penetrating oil to get things cleared up. I then cleaned with break cleaner. Reinstalled the nipple and got a fair amount of dirty brake fluid before it cleared with the bleeder. If you are not getting good flow, you may not be getting all the old fluid and even air may be trapped. Make sure the bleeder nipple flows freely.

[roger0080]

I had bought a 70 K20 with similar problem with disc brakes Dana 44 added to it. Found out they had the pushrod from the petal to the booster too long causing the plunger in the master cylinder to be pushed forward past the rear port. Readjusted the push rod, bled it and all was good. Thought I had a bad master cylinder.

I found out by locking up the brakes (on purpose) on gravel while someone else watched. HTH.

If this was a conversion with everything new as stated, I'd check that too.

[Blazerowner]

I’m pretty sure it’s a cylinder so I ordered two new ones since they are so cheap. Everything is brand new but I don’t put it past these parts to have issues. I will try the motive pressure bleeder again and the vacuum bleed and hopefully the issue will be solved.

[Blazerowner]

It was not the cylinder it seems to have just been air. Vacuum bled the rear this time and real solid now. Weird that the motive didn’t work but it is what it is. Now I have another tool for bleeding. Used a four uncles vacuum bleeder. Literally took 10 minutes to get them feeling solid.

[57larry]

MP Brakes.com has some good tech