Brake issue - pedal slow to return (too weak to shut off lights)
I've searched and read every thread I could find for a solution, but my issue still persists regardless of the fixes I try. This post details my entire history with this problem, so it's quite long.
Issue: Brake pedal doesn't return with enough force to shut off brake light switch. Yes, I've changed the switch. 3 times. Read on for more details.
Truck: 1996 Dodge Ram 2500 2wd 5spd with rear ABS. Vacuum boost, not hydroboost.
Note* - This truck is not my daily driver and gets used very infrequently throughout the year, so some of these steps may have had a long gap in the time frame when I just didn't feel like dealing with the issue.
-noticed issue after new brakes, changed stop lamp switch.
-vacuum test, changed booster, changed switch again.
-changed master cylinder, full fluid flush.
-cleaned combination valve internals, installed new calipers, new rubber lines, new wheel cylinders, full fluid flush, changed switch again.
-issue still persists. I've put a helper spring on the pedal to trigger the switch for now.
I changed the front pads and rotors 2 years ago. I did not open the hydraulic system at the time and the pedal was working as usual afterwards. Some time later, I changed the rear shoes, springs, and wheel cylinder (upgrade to GMC 1 ton cylinder, Napa #37337) and had the original drums resurfaced. I didn't notice any issues bleeding the fluid at this time.
Some time passed when I noticed that my brake lights didn't shut off after having parked. The brakes would release and the truck could roll easily, just the lights were on. I replaced the switch with a Napa switch. The switch was adjusted when the truck was off, but when the truck was running, the pedal would never return itself with enough force to overcome the spring inside the plunger.
Driving the truck into town with the brake lights on constantly is dangerous, so I pull the plunger out until the lights go off naturally (I've heard this damages the switch, but no one's ever been able to explain how it damages the switch yet. Also, the service manual clearly states the switch may be adjusted if needed.) This worked fine until the truck is off and the pedal has been pressed a few times to remove any remaining vacuum in the booster. With the truck off and the vacuum gone, the pedal is very stiff and will return fully. This pushes the switch in to the point where it's too far away from the pedal when the truck is running.
This was a while ago, but I do remember testing the vacuum according to the service manual. I don't remember numbers, but remember being surprised at how strong the vacuum was for such a high mileage truck. I also tested how long it held vacuum with the truck off and it passed that test, too. Also, the pedal always has a couple uses with vacuum left over after the truck is off. The pedal drops a few seconds after firing the truck up and then stops where you would think it should stop.
I went ahead and replaced the booster with a new one from Napa along with another new switch. The new booster's rod was the same length as the old one in the truck. Both the old and the new booster were non-adjustable as far as I could tell, but they were the same length anyways. No change to symptoms. The pedal swings very freely through the entire range of travel when the booster rod is not attached to it.
The original Master Cylinder showed no signs of leakage or defect at all. There was no leakage between it and the booster or at any of the lines. The pedal would always build pressure and stop the truck evenly and consistently. The pedal would not drop when I sat there holding pressure on it, so I had no reason to believe the MC was bad. I replaced it anyways. Bench bled the remanned MC from Napa, installed, and bled the whole system. No change; pedal still inconsistent.
Wondering if it could be sticking from rust or something, I disassembled the proportioning valve and cleaned it carefully with brake clean and a nylon tube brush. It looked good on the inside except for some light yellowing that cleaned right up. It went back together without issue. At the same time, I changed the front calipers, all three rubber lines, and the rear wheel cylinders back to original size. All parts have been sourced from Napa. Remanufactured calipers, new hoses, new wheel cylinders. Bled the entire brake system very thoroughly and installed yet another brand new brake pedal switch.
Problem still persists. The switch gets set to the full retracted position with the truck off, then the pedal won't come all the way back with enough force to close the plunger with the truck on.
At this point, I've changed everything but the hard lines, rear ABS module, and proportioning valve. I've inspected the hard lines and they're dry everywhere that I can see them. There's only light surface rusting in some areas. It's nowhere near enough to have broken through and the area is dry anyways. This problem has existed for the better part of a year now and I've never lost any brake fluid, either.
I'm pretty much out of ideas at this point. There never was and should not be any spring on the brake pedal. I don't want to install one to band-aid the situation, either. If I allow the switch to be in it's fully retracted position, the lights stay on and the pedal moves quite far down before it builds pressure. If I pull the switch out to the point where the pedal shuts the lights off in all scenarios, the brakes drag.
I've noticed that holding the pedal for more than a few seconds results in a weaker return. If I simply press on the brake pedal and immediately release, it seems to return consistently and I can set the switch to that position. But when I hold the brakes (such as coming to a stop on the road), it won't close the plunger at that point. If I park the truck after that and press the pedal a few times till the vacuum is gone, the pedal returns harder and retracts the plunger even farther into the switch.
I'm open to any suggestions, even if you want me to test something I've already tested or changed. Please help. I'm about to give up and take it to a mechanic, but I fear I'll just be paying them to replace all the things I've already replaced and I'll still be stuck with the problem.