StingrayL82 wrote:

One exception to the NO SOLVENT rule, IMO, is if you’re replacing everything but the hard lines, which I am. I’m going to spray the snot out of the steel lines, once everything is off, with BrakeKleen. My entire system, except the steel lines, is being replaced (calipers, cylinders, master, rubber hoses), so I’m not worried about anything being contaminated, given BrakeKleen evaporates.

StingrayL82 wrote:

I’m betting rear brake lights. I didn’t realize the MB300 didn’t have a return spring, until I saw the rear brake lights on. The brake switch doesn’t require hardly any pressure to activate.

Griff in Fairbanks wrote:


Are you talking the brake lights in the rear or the brake light in the dash?

If you're talking the brake light in dash, it's the 'brake failure' switch located on the proportioning valve. It senses the lose of pressure in the front or rear brake systems via a piston 'balanced' between the front and rear systems.

Sometimes this piston gets 'off-center,' yielding false readings. Alternatively, the switch is a normally closed switch, held open by a bump in the piston. If the switch is broken or somehow stuck in the closed position, you'll also experience 'false positives.'

This is the same type of switch, and same function, as the neutral start switch on your automatic transmission. The same with the switches on car and truck doors for interior lights,

(Modern vehicles use magnetic reed or proximity switches to sense 'door ajar.')

Griff in Fairbanks wrote:

'Freeing' stuck or sticky master cylinder pistons:

Before proceeding, consider you're going to use a lot of time and brake fluid to do this. It may be cheaper and faster to replace the master cylinder.

This is temporary fix ... you'll be rebuilding or replacing it anyhow in the near future. Also, you won't know until after you're done if the seals are still reasonably good.

The objective is to use fresh brake fluid to dissolve the old gummy build up.

If the master cylinder piston is totally stuck (won't move at all), you may have a different problem, making all subsequent efforts futile. Likewise, you may wind up investing too much effort or not be able to dissolve the gummy buildup.

Step one is to remove all old brake fluid from the master cylinder and replace it with fresh fluid. It doesn't really matter how you do this. However, you want to make sure fresh fluid gets down where the piston is.

If you're the patient type, who allows things time to work, leave it overnight. (If the piston can move, even slightly, you can help the process by working the piston back and forth a few times every 4-6 hours.

You can try to hurry things up by immediately beginning to work the piston back and forth. But be careful. Being impatient always carries a risk of causing damage rather than fixing things.

If at some point you don't seem to be making an progress, you may need to dump the fluid you've been using and replace with fresh fluid.

Eventually -- hopefully -- you'll get the piston to move freely for the full length of it's travel.

Now, you get to find out if the seals are still any good. More than likely, you've destroyed the seals and need to rebuild the master cylinder.

The above is the exact process I've used to restore very old, very rare master cylinders. In one case, we had to machine new seals, using liquid nitrogen to freeze suitable rubber before and during machining.

I also know, from firsthand experience, that new and remanufactured master cylinders are available for extremely unique applications and vehicles dating back to the 1930's. (While I don't have firsthand knowledge, I believe this applies to vehicles from the 1920's and earlier.)

my440 wrote:

Hi Giff, no talking about the rear brake lights in the back.

Griff in Fairbanks wrote:


I'm going let StingrayL82 help you. I think he has fresher knowledge and I'm kinda jammed up right now, with a half dozen people begging for my attention.

In the case of the aforementioned #$@%#$% '90 Bronco, replacing the switch solved the problem. Age and repeated use cause the mounting bracket and internal mechanism to wear enough so I had to jam the pedal to floor to make the rear lights come on.

In your case, mechanism wear, in the pedal and switch mount, may be causing the pedal to sag or preventing the switch from turning the lights off.

As long as your brakes are working just fine, a light pedal return spring might solve the problem, at least until you can work out a better solution.

my440 wrote:

Griff thank you so much for your info and advice. Much appreciated. Motorhome driven back and forth to propane and sani dump. Will get it sorted sooner or later. May try a spring, temporarily .

Griff in Fairbanks wrote:

Before I duck out of sight ...

A classmate in the early 70's couldn't get his brake pedal to return more than half way without pulling on it manually. He thought he was 'fixing' it when he added a 'shadetree aftermarket' return spring. What he didn't know was the piston in his master cylinder was gummed up.

He discovered his 'solution' made things worse while he was backing out of his driveway ... he pushed on the brake pedal to slow his car and the pedal went all the way to the floor, easy-peasy.

Oh yeah, I forgot to mentioned he'd 'fixed' a problem with his engine by turning the idle up to 900-1000 RPMs. By the time he got clutch disengaged, he was traveling backwards at about 15 mph, with no brakes. Fortunately, a large tree stopped his progress. (Considering he was going backwards, I suppose 'congress' is more appropriate than 'progress.')

We found the pushrod that connects his brake pedal lever (bellcrank?) with the master cylinder laying in the driveway, roughly where he'd been parked. The spring he used to 'fix' the brake pedal problem pulled the pedal back far enough for the pushrod to drop completely out of his car. (In many cases, master cylinder back pressure is all that's holding the pushrod in place.)

my440 wrote:

Griff!! Your a wealth of information. I got to have a look over this all. Thanks kindly. I'm leaning on changing both the master cylinder and booster. Been afew years but just figured I could tweak something to get that dang peddle to go up just a tad to switch off the brake lights.
When the engine is off and the peddle is pushed it returns back enough where the lights go out.
Other than that brakes work perfect. Nice and firm, no spongy peddle, just changed all pads drums calipers.
A work in progress.