Brake pedal drops by itself when engine is started

I have been helping a friend build a 38 Chevy pickup for a good bit now. Was going to try and make the Street Rod Nationals in Louisville, KY but am quickly losing interest in their new "Street Rod PLUS", which is another issue.
The truck is really looking good but I noticed something that has sparked a concern. When the engine is started without touching the brake pedal, it will drop by itself about 1-1/2". It is still not drivable to check the brake function so I can't tell you how the brakes are working for stopping power. It is an aftermarket under-floor power brake setup. The pedal rests quite a bit higher than I would prefer while the engine is not running but as I said, drops to about the right height after the engine is started.
I have never noticed one doing this before without even touching the pedal. Another thing I am concerned about is after applying the brakes while on jack stands and the engine is running, the front wheels will stay somewhat locked. I have to reach down and raise the brake pedal just a bit and the front wheels will release.
Everything is new and it has 2 lb residuals on the front and a 10 lb on the rear. I am going to have to change the 10 lb residual on the rear because I have since put disc brakes on the rear. I am also going to have to change out the proportioning valve because of this.
I don't want to have to put a return spring on the pedal because I don't like it being so high, the 38 Chevy truck cab is already so short your knees are in your chest when sitting in it. I tried adjusting the rod on the master cylinder but then the pedal will go too low to the floor when applying the brake. I thought I would toss this out there to see if anyone had any ideas.

The push rod from the MC may have to be adjusted. But I would see just how well the truck stops when you get it on the ground. Then you can troubleshoot from there.

It sounds to me like the weight of the pedal is not letting the MC get back to the position where it uncovers the port to let fluid flow back from the wheel cylinders into the MC reservoir. Since you don't want a return spring pulling it all the way back it seems that the answer is to weld on a stop tab that limits the return position that you can live with, adjust your pedal to booster rod to unload the booster at that position, then adjust your booster to MC rod to unload the MC at that position and install a return spring to pull everything back to static. You just need to be sure that you retain enough pedal stroke to fully actuate the MC w/o the booster or you could have no brakes with a loss of vacuum/booster failure.

The reason for the pedal fall on start is that the weight of the pedal is actuating the booster, and when you start the engine the vacuum sucks the diaphragm down to a partial applied spot. You need a spring, IMO.

You need that return spring........joe

The return spring on the brake pedal is the final barricade that the oringators of hydraulic brakes had to hurdle back THEN in THOSE WERE THE DAYS!!!!!!!! Don't want to use a return spring-back to the Flintstones

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Originally Posted by jerry clayton

Don't want to use a return spring-back to the Flintstones

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And my feet still hurt!

I have been thinking about it and I also don't see any way to get around the return spring. I guess this is a drawback to having an under-floor mount brake booster. I don't recall ever seeing one that would drop by itself when the engine starts. I can't really adjust the full travel until I get the carpet in the truck, I surely don't want the pedal to bottom out on the carpet before the brakes are applied.
this old truck has been one obstacle over another since the beginning, but it has been built the old school way, back when street rods were built with what was available instead of what can be bought and I am extremely proud of the way it has turned out so far. I need to upload some pictures, I think there are those here that would really appreciate it almost as much as I do. There has been no short-cuts in quality, but has been built with a lot of trips to salvage yards and integrating everything to make it look as factory installed as possible. It has taken many hours of head-scratching, cutting and welding.
My next obstacle is going to be installing the A/C condenser. The thought has come up to use a remote condenser instead of one in front of the radiator. There is room for a condenser but have been having trouble finding a vertical condenser that will fit the bill. Then there is a concern on my behalf of the radiator not cooling the LT1 383 Stroker. The radiator is a cross-flow that my friend had modified and changed tanks on over 20 years ago that now flows vertical instead of cross-flow and he wants to use it because it is already installed and fits. An alternative was a trip to the salvage yard to get a condenser and fan from a late model car that would fit under the bed. I should clarify that a bit. The main reason I really don't want to buy any particular condenser to go in front of the radiator is because, in-case of the radiator not cooling properly, I don't want to have spent a lot of money for a condenser that may not fit a new radiator. If you don't have a great deal of money invested it makes changing/replacing a lot easier to live with.
When we first started disassembling all he had done over the years and changing it he was insistent for it to be built the old school way rather than buying everything. We even bought several factory exhaust systems from 96-97 Cadillac Eldorado's (that are still rust free, I might add) cut and welding them to make the dual exhaust system. It sounds really good and total material cost from headers back is about $63, not including the tips. We cut the original LT1 serpentine belt system down, narrowed it and got it to stuff in between the 38 Chevy truck fender wells, AND, it looks factory and extremely clean. For $130 we purchased a complete rear frame portion from a 98 Chevy S-15 Blazer with a disc brake 373 posi rear end and integrated it into the stock 38 Chevy frame complete with the sway bar and springs and it looks like it is factory. I gotta download some pictures.

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Originally Posted by Hurst01

....I don't recall ever seeing one that would drop by itself when the engine starts.

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To mimic this feature, put your foot lightly on the brake of your modern OEM vehicle and start the engine. Static, the diaphragm resists the light pressure, but as soon as vacuum is applied the pedal moves to match the light pedal pressure. A return spring will eliminate the static pressure, and you can limit the return position providing you verify the total pedal travel is enough for full brake action.