Bad master cylinder ????

[TCombs]

Recently i replaced all brake lines on my 99 K2500 . About two weeks after replacement brake pedal started to fade at stop lights . No problem, replace master cylinder and be on my way , WRONG !!!! I am now on my second A-1 Cardone master cylinder with the exact same problem . I even talked myself into replacing the hydroboost unit , thinking it could be leaking internally giving same results , NOT . So , my now my question is go with new master cylinder ???? I have seen problems with rebuilt master cylinders in the past , but two back to back with the exact same problem ???? Any input ?????

[DmaxMaverick]

This is not that uncommon with Cardone reman's. I have done 4 back to back (for a friend, who insisted) within a month. I finally talked him into an new OEM. 3x the price of the Cardone, but it's been 2 years, and not a peep out of him, and his brakes work. Prices aren't too bad, if you order from an online GM distributor, like www.gmpartsdirect.com (about 50% of list, plus S/H), or get your dealer to deal wholesale (about 50% of list), or jobber (about 75% of list).

Just for future reference:
Your hydroboost unit will NEVER cause the brakes to bleed down. When they fail, they will cause less boost (more pedal pressure required), pulsing with PS pump surge/squeal. In the rarest of cases, the input rod can break or deform, causing a grinding sound during pedal pressure. But they can never cause a bleed down. The boost and MC systems are not hydraulically connected. The units are just too expensive and difficult to R/R to replace unnecessarily. Most replacements are premature.

[john8662]

Another thing is the method you use to prepare for instalation.

You have to prime the new master cylinder by the use of plastic plugs on the outputs and manually pumping the cylinder.

Pump until primed, it takes a LOT of time to finally get it to stop expelling bubbles.

Also, the method of bleeding the brakes matters too. I get a helper to press and pump up the brakes. On a dry system (recently had the joy of this), just pump up the best you can, and then have the user hold the brakes. Just open the petcock and let it all dribble out until it stops. Then, close the petcock, pump up, repeat until you get no bubbles. This doesn't require a clear line on the petcock either, just l have a catch container for the fluid below (oil draining pan works).

J

[tommac95]

I vote with DMax here. I had occasional/intermittent LOW PEDAL (!??!) on the brake for a couple rebuilds over , like 6 years.
Last fall i installed a new OEM Delphi MasterCylinder , and it has been smooth/SECURE EVERY STOP since , no remaining pedal-height issue, either. By that -"OEM"- i mean, i ordered a new one from RockAuto.com , which came in a sealed plastic bag in the Delphi box.
I thought the Cardone/Delphi affiliation would work out nicely ... but most of their rebuilds are filthy. I'm sure they're trying to comply with environmental/emissions regs , but cleanliness DOES matter in mechanical systems.

Not to rant endlessly , but this was an issue that i followed/posted some threads here concerning same, in past years . When i removed from service one of the remanned Cardones' , i dismantled it ... and noted that more than one of the 'rubber' cups/rings had embedded in them MULTIPLE bits of metal flakes [not great sealing characteristics].

I wasted about three times the value of a new OEM MC in screwing around -- and that says nothing about the danger of 'near-NO-stops' , and the puzzling distraction. Further i noticed that BOTH MCs and brake pads can function well at one level of wear/adjustment , but mess up significantly when worn down a bit ....

And , it pays to clean and give a good primer coating to the mating surfaces of the ID of the rr brake drums , and the OD of the rr hub/axle-tube.

[TCombs]

After the second A-1 rebuilt master cylinder and the helpful replies i recieved , i got a new master cylinder from the parts store . Although it wasn't an AC Delco , it was a new master cylinder from Raybestos . As soon as i installed it on the truck , after bench bleeding , i bled it on the truck and the difference in pressure at the lines was unbelieveable !!!! The brake pedal feels as good or better than ever . I have had problems in the past with A-1 rebuilt master cylinders , but for some reason forgot on my own truck !!!!! Thanks for all the replies !!!!!