Brakes lose pressure

Need a little help with this one... 1996 chevy 1500 5.7 engine 33,000 original miles. Truck doesn't drive long distance but is driven. Ok problem is brakes have good pedal when pressed but slowly start losing pressure while applying brake to the point the pedal will eventually end up on the floor. The braking is there but the longer you hold them the further the pedal will go. Now here is what I've done. Changed master cylinder and brake pads. Bled brakes. I didn't have a scan tool to cycle ABS so I did the gravel road trick to cycle the ABS and then bled again. There are no leaks and no brake warning light on in gauge area. What would cause the pressure lose. Also this was a problem before anything was touched or changed. Thanks for any help.

Sometimes when the hoses get bad they will expand. With the vehicle on a jack stands use a flashlight and watch each hose with someone else pushing the brake pedal. You may have gotten a bad master cylinder. Look for a bulge in the hose.
If this doesn't work you can pay someone with a scanner to bleed the brakes.

Need a little help with this one... 1996 chevy 1500 5.7 engine 33,000 original miles. Truck doesn't drive long distance but is driven. Ok problem is brakes have good pedal when pressed but slowly start losing pressure while applying brake to the point the pedal will eventually end up on the floor. The braking is there but the longer you hold them the further the pedal will go. Now here is what I've done. Changed master cylinder and brake pads. Bled brakes. I didn't have a scan tool to cycle ABS so I did the gravel road trick to cycle the ABS and then bled again. There are no leaks and no brake warning light on in gauge area. What would cause the pressure lose. Also this was a problem before anything was touched or changed. Thanks for any help.

the vehicle must have a leak. since it does brake good with short time on the pedal I would guess it is a small leak. all the brake lines will need inspecting. the line running to the rear is a big failure. could also be brake cylinder in the rear. small leaks are not going to drip to the ground.

If the fluid level is not receding I would look at the master cylinder again.

Been working on in my limited spare time. I've inspected lines from front to back. Changed calipers. Changed rear wheel cylinders. The cylinders were wet on the ends. Bled brakes twice just to make sure no air was coming out or any small air bubbles in fluid. Still same exact problem. Going to try and bled again when I get some time off and if that don't work the I'm going to try and change the new master cylinder out again.

Been working on in my limited spare time. I've inspected lines from front to back. Changed calipers. Changed rear wheel cylinders. The cylinders were wet on the ends. Bled brakes twice just to make sure no air was coming out or any small air bubbles in fluid. Still same exact problem. Going to try and bled again when I get some time off and if that don't work the I'm going to try and change the new master cylinder out again.



with this type drum brakes if your drums are worn or cut , the brake cylinders come out a lot more than they should . then when you replace those drums/shoes the damaged exposed metal causes these leaks.

so the leak was the rear brake cylinders I would expect you may need to bleed this s a few times. I would lift all wheels off the ground and start at pass rear and bleed the brakes several times. start vehicle and then bleed with high pressure.

with these long time leaks air gets into the system much harder to remove it all.
hopefully you did not allow much brake fluid to leak out on the part replacing. The use of clear tubing on the bleed fitting is required to see if the fluid is clear after several bleed cycles..

On rear drum brakes sometimes you need to use the manual adjuster and tighten until the rear shoes just slightly rub. Lose rear shoes will give the feeling that you have air in the system.