Low peddle

I have a 94 f150 xlt I put new front rotars and a brake caliper driver side bleed them when truck is off peddle good hard stiff once u turn on truck peddle does down to floor what could be cause of this

my suspicion would be a bad master cylinder. Sometimes, the act of bleeding the brakes (pumping the pedal) causes a tired MC to give up altogether. If you pump the brakes repeatedly, do they firm up some? That would confirm the diagnosis.

Sure you got all the air out ? Did you have a pedal problem before doing the brake work ? If not I would liberally bleed all 4 wheels . Brake fluid needs changed ever so often anyway .

Going with the bad MC. Once the truck is on you have the power brake assist. This pushes fluid past the seals in the MC.

I’m betting on the master cylinder too.

I’d also suspect a bad MC, but with some caveats.

Normally, the pistons in the master cylinder chambers travel a path not completely through the chambers. In the process of bleeding the brakes after a caliper change, the rubber seals on the MC pistons can travel to areas in the chambers that they don’t usually go, and this can cause failure of old seals. Normally I’d expect that failure to occur without the added force of the booster on the center shaft, but it’s possible that in this case the added force was enough to push the seals into failure, meaning they allowed the brake fluid to flow past the piston seals rather than transferring all the force down the line to the calipers.

A couple of caveats, however. If your brakes pumped up to “stiff” before you turned the engine on, you may simply still have air in the system. If they’re soft at first and get hard when you pump them, that’s a sure sign that the air is being compressed and the system needs to be bled again. If you have this symptom, start with the caliper farthest from the MC and bleed the entire system.

Another caveat is that what you’re feeling might be normal. A normal quick-check of the function of the power booster is to (1) with the engine off, pump the brakes until the pedal hardens. Then, while pressing on the pedal, start the engine. The pedal should sink (NOT to the floor, but noticeably) and soften as the diaphragm in the booster assists you in pulling the center brake rod that goes from the pedal, through the booster, and into the master cylinder. If this is what you’re feeling, if the pedal isn’t actually going to the floor, test the system again. If it now acts normally, that might have been what you felt.

I’m the type of person that when given a problem that can have multiple causes , I start with the simplest , easiest & least expensive things first . For instance if I have a bulb that doesn’t burn I check the bulb & the fuse & whether voltage is getting to the bulb before I start ripping out wiring . Nothing was said about pedal problems before the brake work . We know a caliper was replaced so more than likely air was introduced into the system so I’d start with a complete system bleed . After I was convinced there was no air left in the system I’d consider the master cylinder & I agree it could be a master cylinder problem but I wouldn’t start with the master cylinder .

Good points.
You also want to be sure the right caliper was used on the correct side and the bleeder is on the top. I’ve seen a caliper put in with a bleeder on the bottom, and you cannot bleed the system that way. You’ll always have an air bubble in the caliper.