Urraco454

Hello,
i recently purchased a 1981 5spd that had been sitting for 4 years. I got it running but the brakes were rusty and in poor shape so I rebuilt the calipers and did new pads and rotors in the the rear along with a couple brake lines. Put it all back together and bled everything and no brake pressure with brake pressure light on dash. Figuring I likely had a bad master cylinder I went ahead and replaced it as well. I bench bled the master as well as I could and bled all the brake lines again, this time with a mityvac. Still no brake pressure and light remains on on dash. I was not able the bleed the clutch as the bleeder valve broke off but ironically the clutch still has pressure and works. I tried clamping the brake lines at each caliper to see if maybe one of the rebuilt calipers was the issue with no change. No obvious leaks I can see. I’m at a loss as to what to look at next. Could it be the pressure regulator or booster? Any help for a new 928 owner would be much appreciated!

Jason89s4

After all of the procedures that you have done, check your brake fluid reservoir level carefully and then when running the car (brake booster connected), and pump the brakes a dozen times or more. Do you have any loss of fluid in the reservoir? You stated no obvious leaks, but lets determine if the brake fluid is going somewhere it should not be, or if you just have a bleeding/air bubble issue.
-Jason

Mrmerlin

What fluid are you using
what brand of MC did you get
did you look inside the booster for fluid
its a good idea to replace all 4 lines with stainless flex lines don’t crimp the lines as you can damage them internally
bleedin procedure is
start at MC crack the b nut closest to booster let it leak till no air bubbles gravity
the move forward to the tip
if it has a tip bleeder do it last then move to the proportioning valves crack those B nuts
once this is done get a helper to foot bleed the MC start the procedure you just did except with the foot pedal being pressed

Then move to the wheels
LF RF LR RR in this order
i suggest use ATE fluid

Petza914

Also if the brake pressure sensors that are screwed in upside down to the dual channel master cylinder on the inner driver's side fender get wet, they will throw the dash error. With them being upside down, the rubber boots that cover the electrical connections on the bottom trap water. You have to disconnect the battery to get them to reset, otherwise, even when you fix the pressure issue, you'll still have the error.

Mrmerlin

I suggest to use DC111 on the tank grommets and the pressure connector boots to repel water

Urraco454

Thanks to everyone who has replied so far! I worked on the car again today and confirmed there is no loss of fluid from the reservoir after operating the brake pedal. I attempted to bleed the master by cracking open the lines. When operating the brake pedal I can see bubbling at the rear port facing the passenger side and at the rear port facing the driver side (opened at the pressure regulator as I cannot reach that side with the reservoir on. After an hour of pumping I was still seeing air bubbles. I tried reverse bleeding from the calipers then normal bleeding. There is maybe a very slight improvement in the pedal but still nowhere near driveable. I suspect there must still be air in the master cylinder but I have no idea how to get it out at this point. I did rebuild all the brake calipers, is it possible to get this kind of issue from a caliper? The rebuild seemed really straight forward but it’s possible I could have made a mistake. Thanks!

Mrmerlin

please answer the questions in post 3

Urraco454

Hello,
No brake fluid in the booster.
Master cylinder was a Centric brand (although it actually has ATE stamped on it).
i’ve just been using a generic DOT 3 brake fluid.
I did replace the the front driver hard line and flex line to the caliper (not a stainless flex line though)
i do have replacement flex lines for the other calipers but I haven’t installed them yet as the connections to the hard lines are very rusted and I was worried about creating another big job if I ended needing to replace all the hard lines.

Thanks!

Zirconocene

For what it's worth, I'm in a similar situation, but have not yet replace my MC (that's job for this long weekend). Dr. Bob gave me a great, detailed write up on bleeding that's worth checking out:

https://rennlist.com/forums/928-foru...tem-flush.html

Cheers