88 Mighty Max SPX 2.6L - No Brake Pressure

Hi everyone, new to this forum! Have an 88 Mighty Max SPX 2.6L that I've had for a few years now. Have had a few brake lines replaced and last summer had one rear load sensing valves replaced. Having an issue now with the brake pedal going to the floor when the truck is running. When the truck is off, I have tight pressure on the pedal. Could this be the master cylinder and booster going? Brake fluid must be leaking somewhere becuase I topped it off at the cylinder a month back and its low now. Don't see any spots on the driveway. Any ideas? Thanks

Likely the master bypassing. Time for new.

Welcome to the site. Remove the rear drums and check the wheel cylinders. They can leak quite a bit before it becomes visible on the wheel or backing plate.

The brake fluids' got to be going somewhere. The highest probability is the rear brake cylinders as suggested. Next is the master cylinder - the internal seals can wear out and brake fluid can bypass when you hit the pedal. If the booster is faulty, the engine idle will creep when you put your foot on the brake (sometimes there's an accompanying whistling noise - also check the booster hoses and one way check valve for air leaks). A brake hose may be the next fail point due to age as they can expand way more than they should.

Thanks everyone, looking at it again today I noticed a a fresh drip of brake fluid on the booster below the master cylinder where they connect. Going to replace the cylinder to start and go from there!

Good eye. Hopefully that fixes it.

Yeah I forgot this is one place that is difficult to find a leak - the rear of the master cylinder. A rebuild/replacement will fix your braking woes :)