Soft brake pedal

[Nima]

Hello:
I am in the process of my LS swap conversion on my 1953 truck and I am having issue with soft brake pedal. Here is the history:
The 5.3 engine is from a 2001 silverado and the the brake booster and MC are form a 2001 suburban with the same size engine. I have bled the brake many times, the pedal firms up when the engine is off but as soon as the engine is turned on, the pedal gets very soft and goes to floor if pressed. YThe pedal has the feedback, it doesn't stay at the floor but you can just very easily push it to the floor with hand, no resisatnce? I thought the booster was an issue (junyard sourced for the mockup purposes) but when I changed that with the donor vehicle (which was running and stopping fine) I still have the same issue.
The MC was bench bled ( Ihave done these few times, not that I am an expert though) when installed, I don't have any leak from the lines that I can see. I have a mityvac bleeding system which I have been using.
Thanks for the help

[SCOTI]

Originally Posted by Nima

Hello:
I am in the process of my LS swap conversion on my 1953 truck and I am having issue with soft brake pedal. Here is the history:
The 5.3 engine is from a 2001 silverado and the the brake booster and MC are form a 2001 suburban with the same size engine. I have bled the brake many times, the pedal firms up when the engine is off but as soon as the engine is turned on, the pedal gets very soft and goes to floor if pressed. YThe pedal has the feedback, it doesn't stay at the floor but you can just very easily push it to the floor with hand, no resisatnce? I thought the booster was an issue (junyard sourced for the mockup purposes) but when I changed that with the donor vehicle (which was running and stopping fine) I still have the same issue.
The MC was bench bled ( Ihave done these few times, not that I am an expert though) when installed, I don't have any leak from the lines that I can see. I have a mityvac bleeding system which I have been using.
Thanks for the help

I'll go out on a limb & question your pedal ratio here. You need to know/understand the pedal ratio requirements for the donor combo & make sure you're as close to that (or exceed) for the best results. You might want to research the need of relocating your pedal pivot point as one possible fix.

This is just one possible avenue as I'm not very familiar w/the pre-C10 trucks. I know when I removed the factory installed hydroboost set-up on my Squarebody dually for a same generation/model vacuum based brake set-up from a Suburban, the pedal ratio had to be changed as it was killing master cylinders. Once I corrected the ratio, my master cylinder failures stopped. You should be able to research the info or possibly ask others w/similar year model trucks that were converted to get an idea of what worked for them.

[Nima]

Thanks for the suggestion.
I just checked the pedal ratio on the donor which is about 4.6" and on my 53 is about 4.3". I am not sure if 0.3" makes that much difference, does it?

[SCOTI]

Originally Posted by Nima

Thanks for the suggestion.
I just checked the pedal ratio on the donor which is about 4.6" and on my 53 is about 4.3". I am not sure if 0.3" makes that much difference, does it?

I wouldn't think so. I'll try to look up mine again to see what the difference was (87 3500 w/hydroboost vs 87 2500 Burb w/vacuum).

[Joyridin]

So this has a hydroboost or a vacuum booster?

[Nima]

This has vacuum booster which is matched to the same engine type (5.3) and model years.
So, I managed to get some brake!
So, the problem (confusion) that I have been faced with is: there is no fluid in the lines!! I changed the junyard master cylinder with brand new (not refurbished) ACDelco with no luck and the same problem: hard brake pedal when the engine is off and soft pedal to the floor when the engine is runing.
When I was taking the old master cylinder off, I noticed that when I discounted the brake lines from the MC, there was no brake fluid leaking/dripping out of the MC, eventhough intially I was getting brake fluid at the calipers when I was vacuum bleeding. I didn't think much of it at that time. I thought I got lucky and it is not making a mess! Besides, the level of the brake fluid was not very high in MC.
Fastworward, bench bled the new MC, installed and connected the lines but the same problem. The diufference was that this time around, I was not getting any fluid at the calipers when I vacuum bled them!!. I thought I had a clog some where therefore I took one connection of brake lines apart at a time apart and moved upward. There was no brake fluid leaking/drippg out of the brand new MC?!! I bled the MC again while on the truck, when I push the pedal I get nice flow of the fluid but otherwise the orifices are sealed thight with no brake fluid is coming out of them! So, I connected all the lines back, got a body of mine to press the pedal and keep it pressed to keep the MC vlaves open, and I vacuum bled at the calipers. Now I have a brake although it is still very soft. I believe I still have some air in the lines.
So far the lesson learned: this type of MC does not allow the fluid passage if the pedal is not pressed which means it won't allow traditional vacuum bleed!
What do you guys think?
Sorry about the long post