308 QV Brake Failure light confusing me

Hi guys,

I have searched this forum, but have not found a good answer to my problem. I recently had a seized RR caliper and performed a rebuild (using Birdman's excellent tutorial). The pistons were seized and there was some corrosion.

After the rebuild, I proceeded to bleed all calipers until all air came out. I drove the car and all was fine for the first 20 miles or so. When I drove the following day, the "brake failure light" came on and I suspected a leak. This was not the case. In fact, on my way home the light turned off and all felt normal again.

At home I removed the wheels, checked the system for fluid leaks and did not find any. I attached my motive power bleeder and pumped it to 10psi. I let is sit for an hour and there were no leaks. The system held pressure too. At this point I decided to bleed the brakes again, just in case. A few small air bubbles came out and all seemed ok.

The following day the light came on again! I have traced the problem down to the differential pressure switch, because the light will go out when I disconnect it.

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I gravity bled the brakes again, but the light won't turn off. When the brakes are applied, all four calipers build pressure and the pedal feels fine. What is happening here? Can it be that the blue plunger was moved in one direction due to some air in the system and got stuck in place? I really don't think that there is a pressure issue with either circuit and there are no leaks at all. Also, when I bled the brakes, any pressure differential should have been equalized, right? Do these switches get stuck? The springs should return them to the center. Has anyone else had this issue?

Thank you all for any suggestions.

Jan

 

The piston (colored blue) got slightly offset one way or the other during your repair and bleeding. Its designed to shift one way or the other in case of fluid pressure loss in one of the circuits. It will not recenter itself. It will take a little patience but easy really to fix. Hook a bleeder bottle to a caliper...take your pick. Open the bleeder with key on and push pedal down all the way and when it stops push hard for a second. That will have moved piston one way or the other. Close bleeder and with key one see if light is on. Then push on pedal and see if light is on. If its on or comes on you probably guessed wrong on which end to start at. If its off you probably just fixed it. If its on go to other end of the car and repeat the process but if 1st stroke makes no difference try a second. That should take care of it. I have seen those combination valves get rust or other debris and just need replacement.

 

A number of u=years ago I had a similar problem when a front caliper froze. Upon application of the brakes the pedal got hard much higher than normal, braking was significantly compromised and the warning light would come on. Upon release of the pedal the light would go out and the pressure in both branches would equalize. Apparently in my case the springs in the switch were sufficient to re-center the blue piston. After repair I had no problem once the system was correctly bled.

If your light is coming on after the repair but the brakes feel ok, and/or the brakes feel ok and the light remains on after you release the pedal, then I would suspect the switch is clogged or stuck. If the brakes feel different when the light come on I would suspect that one of the calipers is still hanging up intermittent.

Lastly, make sure you set the clearance correctly on the rear calipers, 0.1 mm.

Also make sure the brake fluid is full in the master cylinder as low fluid can also trigger the warning light.

 

Thank you for the advise! I’ll post a reply when I have fixed it.

 

I just fixed this concern on my 1981 308 by using Rifledriver’s bleed procedure. The front calipers were rebuilt right before I bought the car and it’s said that improper bleeding is usually what pushes this valve to one side, so that’s probably what happened to mine.

 

Well, I tried the procedure twice (both the front and the rear twice and alternating) and the light is still on. I’m wondering if I have a problem in the master cylinder because the pedal felt a little “sticky” when you first push on it. Needless to say that the pedal feels different with the engine off anyway.

Both the front and rear brakes build pressure and the car does brake well when running.

Could it be that the little piston is jammed/seized in place? I suppose I could take the pressure differential switch out and clean it before moving on to a new master cylinder.

I’m also thinking about driving it for a while to see if it comes unstuck with some use. Thank you all for your suggestions. Rifledriver’s method seems absolutely logical to me and I’m puzzled why I can’t get to move.

 

If the brakes work correctly and the light is on constantly then either the pressure switched is stuck, the fluid level in the master cylinder is low, or the switch that detects the fluid level in the master cylinder is stuck. It's not uncommon for the fluid level switch to become stuck if the reservoir is emptied and then refilled. You can test this by disconnection the two wire to the switch on top of the reservoir and leave them loose or short them together. If the switch is the problem either looses or shorted should cause the light to go off. If the light stays on either way, I'd try the same thing at the pressure switch before replacing the MS.

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When I recently changed the brake fluid on my '89 328 (ABS) I failed to top up the master cylinder completely, observing (and thinking) there was plenty of fluid in the reservoir. But I started getting the warning light under certain conditions. So I added a bit more fluid and that took care of the problem. Apparently, the "low warning" point is not far below the "full" point.

 

gjv: if your light’s not on all the time then the valve is probably not stuck, just shifted. If the circuit’s not being grounded by a fluid level sensor then your problem may still be in the brake failure warning valve. I removed the sensor and (with a mirror) was able to see that the valve was shifted to one side. I started by bleeding one of the lines opposite the side the valve was shifted to and was able to move the valve to the other side (again, confirmed with a mirror). Once I knew that I could move the valve then I reinstalled the sensor, turned on the key, cracked open a bleeder, and pressed on the brake pedal until the light went out.