Brakes issue

[SamoYU]

Hello

I have a problem with my Wd21 2.7td. The issue is with brakes not working properly. When the engine is not working the brakes are firm and the pedal doesn't go too far down, but when i start the engine the pedal goes almost all the way down and the brake light on the dash shows up. If I press them a few times the pedal gets hard but only for a second then drops down again and the light is showing 6all time. I have a new master cylinder, rubber hoses, brake calipers, rear brake cylinders, brake pads, rotors and brake shoes. I have bled the brakes more times than i can count and there is no air inside. What can be wrong?
Thanks for all the help.
 

[gamellott]

Sounds like a bad master cylinder to me. I had something similar happen to me in an old Ford. I don't know how prevalent NEW parts for that are, but you're better off getting brand new vs rebuilt if available, and not something from China. Also, you could possibly get something from a different vehicle. Not a WD21, but I got a master cylinder from a 200SX and put it on my 210, and worked fantastic!

[Slartibartfast]

Did you replace all that stuff and then have this issue, or did you replace all that stuff while chasing this issue?

 

The pedal does normally get softer when the engine is running (that's the booster at work), but obviously the pumping up and bleeding down ain't right. Bad seals in the master could cause the bleeding down, but unless it's leaking externally (or into the booster), draining the reservoir, I wouldn't expect that to set the BRAKE light.

 

I would start by checking the two switches that control the BRAKE light. There's a low fluid switch in the brake fluid reservoir (float with a magnet, reed switch underneath), and there's a switch on the side of the parking brake lever (might have to remove the console to check on that one). Make sure the switches are good, the parking brake is off, and the res is up to the full mark on the side. If you're not sure, unplug them one at a time, see which one makes the light turn off.

 

The BRAKE light can also come on with a couple of other lights (battery and "AT oil temp" if it's automatic) if the alternator goes out, but if it's just the BRAKE light that's on, then it ain't that.

 

From your other posts, it looks like you've got a diesel. I don't know how the power brakes work in the diesel trucks, but I would not be shocked if the vacuum system for the booster had a pressure switch controlling a warning light--possibly that same BRAKE light. I don't have the right service manual to check that. If your parking brake and reservoir switches check out, let me know and I'll dig out my '87 manual. It covers a couple of diesel engines, maybe it covers what's different with their brakes.

 

How did you adjust the rear brakes after installing them? My understanding is that you want to back off the parking brake adjustment, adjust the shoes with the star wheels, and then adjust the tension on the parking brake cables. If the shoes are held by the parking brake cables instead of the star wheels, I'd expect a bunch of free play between the wheel cylinders and the shoes, resulting in excessive pedal travel--maybe enough to require pumping? Again, though, I wouldn't expect this to give you an idiot light. I don't think it would explain the bleeding down, either.