Brakes Strong At 1st, Then Pedal Goes Down

[bhbingle]

95 ES300 brakes have been giving me issues for a while. I have replaced the pads, calipers (front), all brake fluid, and bled them twice. The brake are STRONG when I first apply them, but then while "sitting" there the pedal goes down and brakes are not as strong.

I have a repair manual and have run through some tests on master cylinder and booster....nothing indicates an issue. Any suggestions?

Thanks in advance for your help!!!

[George_Jetson]

Is the pedal slowly going to the floor if you sit there, with no lose of brake fluid? If so you have a bad master cylinder, it is leaking internally.

[bhbingle]

Is the pedal slowly going to the floor if you sit there, with no lose of brake fluid? If so you have a bad master cylinder, it is leaking internally.

I wouldn't say it's "slowly" going to the floor, but there is definitely no fluid loss.

Is there a way to test the Master Cylinder while it's still on the car?

Thanks for the reply, GJ!

[George_Jetson]

If the pedal is going down while you sit, without any lose of fluid the master cylinder is about the only thing it could be. The key is the no lose of fluid. This means that the fluid is leaking past the seals inside the master, and re-entering the reservoir. Now if it was losing fuild, that would be a entirely different thing, many different components could be leaking and causing the pedal to go down. The master cylinder is the only one that can allow the fluid to re-enter the reservoir. This is a fairly common failure mode. One other mode is when you are losing fluid, but you have no visable signs of a leak. In this case the fluid is going into the brake booster.

There are no other tests to isolate it, (that I can think of). Make sure that you bench bleed the master cylinder, before you install it into the car. It is very difficult to properly beed a master while it is installed in the car.

[bhbingle]

If the pedal is going down while you sit, without any lose of fluid the master cylinder is about the only thing it could be. The key is the no lose of fluid. This means that the fluid is leaking past the seals inside the master, and re-entering the reservoir. Now if it was losing fuild, that would be a entirely different thing, many different components could be leaking and causing the pedal to go down. The master cylinder is the only one that can allow the fluid to re-enter the reservoir. This is a fairly common failure mode. One other mode is when you are losing fluid, but you have no visable signs of a leak. In this case the fluid is going into the brake booster.

There are no other tests to isolate it, (that I can think of). Make sure that you bench bleed the master cylinder, before you install it into the car. It is very difficult to properly beed a master while it is installed in the car.

Thanks very much for the advice. Will put it to use this week and get this thing fixed! Much appreciated.