TarVette1

I just finished changing out my brake lines and bled the lines using 2 people. Pumped up the brakes and bled them until I had no air. Furthest caliper first and so on. Brakes are still mushy and after I pump them up I can feel them slowly bleeding off, Do I have a problem or am I just doing it wrong??? 2001 C5

mneblett

As a general rule, if the pedal isn't holding pressure (and eventually will go to the floor) it's because the seals in the master cylinder are letting brake fluid leak back into the reservoir -- in other words, you have an internally leaking master cylinder, and it needs to be replaced.

I've seen this in motorcycles, where water causes corrosion in the rear-most portion of the master cylinder bore (the portion of the bore that the master cylinder piston doesn't reach during normal brake operation). If there is a subsequent brake bleeding exercise where the brake pedal is stroked to the end of its full travel, as the master cylinder piston seals sweep into the rear (corroded) portion of the master cylinder bore that they don't normally see, the corroded bore wall surfaces cut up the outer edge of the piston seals. The seals will never thereafter hold pressure (and hence the pedal slowly sinks to the floor).

Sorry to be the bearer of bad news; I hope someone chimes in and shows I'm wrong, but based on what you've posted I'll be surprised if it's not the master cylinder. Not that hard to replace if that is it.

Patches

2001 and later C5's: Bleeding sequence changed from the traditional RR-LR-RF-LF to RR-LF-LR-RF.

If after repeated attempts this still doesn't help, you may need a pressure bleed with a Tech II at the dealer as you may have air upstream of the ABS and the power bleed is needed to actuate the ABS unit and purge the air.

Bill Dearborn

When replacing the brake lines did the brake fluid disappear from the master cylinder reservoir? If so you may just have the problem indicated here. It is unlikely not following the bleed sequence would cause the problem you are experiencing but if the brake fluid level dropped below the level of the BPMV you probably have air in the unit and need to see somebody with a Tech 2 to get the ABS system bled.

Bill