Brakes work but pedal sinks if I apply constant pressure

[95wtlb]

The brakes seem to work fine stopping the truck, but if I'm idling at a red light and put steady pressure on the brake pedal, the pedal will slowly sink pretty much down to the floor.
I did the booster test and got a good hiss when removing the vacuum hose after the truck has been off for a few minutes. So I guess the booster is ok.

Advice please. Thanks.

[nice06z71]

check for any brake fluid leaks, calipers, lines, wheel cylinders (if you have drums) anywhere really. If none are found it could be an internal master cylinder leak and may need to replace it.

[tjk1812]

master cylinder might be going bad

[Tweaks]

Classic master cylinder failure symptoms. Just give the brake system a good inspect for leaks before you head off to get the replacement master cylinder. Start by checking the fluid reservoir. If that is full then there a probably no leaks. If no leaks are found, then replace that master cylinder with confidence and your problem will be gone.

[95wtlb]

Thanks everybody. Is there a specific test I can do to test the master cylinder? Or does everyone agree with Tweaks when he said if there are no leaks I can "replace that master cylinder with confidence"?

[vekster101]

A leak can be pretty small and hard to spot....doesn't always drip.
Take off the wheels have a buddy hold the brake peddle down and walk to all 4 wheels looking for any wet spots. I had a leak at the caliper and flex line meet, the washer wasn't seated properly and it just seaped (sp.) out and did not drip but did cause the peddle to sink.

[95wtlb]

Originally Posted by vekster101

Take off the wheels have a buddy hold the brake peddle down and walk to all 4 wheels looking for any wet spots.

Maybe a dumb question here, but would it be ok to only take off two wheels at a time (I only have two jack stands)?

[dave89iroc]

Originally Posted by 95wtlb

Maybe a dumb question here, but would it be ok to only take off two wheels at a time (I only have two jack stands)?

yes, hell, you could do one at a time if you wanted

[Tweaks]

Originally Posted by 95wtlb

Thanks everybody. Is there a specific test I can do to test the master cylinder? Or does everyone agree with Tweaks when he said if there are no leaks I can "replace that master cylinder with confidence"?

I have experienced leaks with brakes and clutches. The systems work fine up until the point where the reservoir level gets low enough to let air into the system and the pedal becomes soft or spongy and eventually goes to the floor and won't pump up. If you are sitting at a red light and holding normal pressure on the pedal and it slowly goes the the floor, but pumping the pedal brings the brake power back to normal temporarily, then this is a good indication that the master cylinder has an internal seal that has gone south and the part needs to be replaced. My brother's Jeep did this for years before he finally replaced the master cylinder, which was a simple do-it-yourself ordeal.