Really strange problem

[wmhjr]

My buddy has a custom '53 Ford truck. G Body front sub, tilting front clip, sbc powered. The brakes are different. Master and booster are '77 Granada. Rotors and calipers up front are '77 Monte Carlo. Everything used to work fine.

He had some issues which resulted in having to replace the master cylinder. Now, when the engine is not running, the brakes are bled and pedal is extremely firm. Can't get any more air out of the system. But when you start the engine, pedal goes almost to the floor when you step on it before producing any effort. Again, lines have been bled more times than I can count. Have tried several new master cylinders. Replaced flex lines and calipers. Old combo valve is gone, replaced with prop valve in front of rear brakes (drums). Booster has been inspected and tested by brake shop.

Any ideas?

[CamaroAJ]

did he bench bleed the master?

[wmhjr]

Yes. Master was bench bled (each time).

[JRouche]

Wow. Dejavu or what ever that word is. It seems like there was another post with a very similar issue bout a year ago. And the replies went on and on and there was still a problem. I dont remember what the fix was or even if there was one.

I think it lead to a faulty MC. Cause basically when you bleed the system its a hard physical contact. The MC rod from the pedal is in firm contact with the MC plunger. If you have a firm pedal during bleeding the application of vacuum to the booster does nothing more than apply more force.

When I have had a bad MC it would bleed internally and the pedal would drop to the floor. But it did it even when bleeding with the engine off. But that was on a small bore MC for an import car. Small MC. And the small bore applied more pressure internally. It only needed a small booster because the car was light and could use a small bore MC. So the internal leak showed up during bleeding.

But you would be surprised at how much more the booster helps the pressure from the pedal. And maybe in some cases, with large bore MCs the internal leak cant be overcome until there is that extra help from the booster. Just a thought. Oh, look through some of the older posts, bout a year ago, for the same issue. I cant remember if he came up with a fix. I think it was the MC though. JR

Oh, and an after thought. How much fluid did he pass through the system after replacing the MC. I have found it takes more than I thought. On one car I had fluid coming out the rear, no bubbles after the rush of bubbles that I thought was from the MC replacement. I ran a few more strokes, still clean and buttoned it up and moved to the other bleeders. Problem? I had a soft pedal on start up. What!

So I bought another quart with the intentions of running ALL of it through cause I dont like to use opened bottles. So all of it was going through.

After cracking the rear bleeder and pumping it through it was still clear, until. Yup, I got a massive train of bubbles that were still in the lines. I didnt run enough fluid through the first time or they dislodged. And really, I was happy to see the bubble train. That told me that was where my problem was. I ended up running the entire quart through the lines, got more bubbles on the other rear bleeder and none through the fronts.

I dont think thats your friends problem. Pedal to the floor means a bad MC to me if the system is bled.