My 98 just developed (again) a low brake pedal. I know this indicates either a leak or air in the brake system. The brakes DO work ok after the pedal sinks down a bit. Ok enough to drive around the block, but not safe to drive on the highway. Obviously.

The previous owner installed a new master cylinder not long before I bought the truck. It had low pedal then. I bled all the caliper/wheel cylinders and it did bring the brake pedal up (some) and it was stiffer.

I should mention all the pads/shoes/calipers other hardware are in good shape.

However, if I hit the brakes HARD, the pedal would go low and the indicator lights on the dash would come on. The fluid level was fine and there is no indication of brake fluid leaks anywhere. Pumping the pedal up and repeated hitting the brakes (in a parking lot) brought the pedal back up again.

Now that the pedal is low again, I'm thinking their is still some air trapped inside the master cylinder.

I'm going to pull out the MC and bench bleed it and see if the fixes the low pedal.

Will I need to rebleed the entire brake system after I remove/replace the master cylinder ?

 

Yes.

From your description of the brake indicator light coming on you are likely tripping the differential pressure warning switch. I'd start with thoroughly bleeding the system first - before pulling the master cylinder.

Note: you may not be able to bleed the system yourself. If the system has been opened you need to cycle the valves on the ABS to eliminate all the air. You need a DRB or a scanner that can actuate those valves to do it.

 

I had about the same problem with an old Corvette. No leaks anywhere but it would keep losing pedal. It was the calipers. What would happen was they would draw air in around the caliper seals but not enough to leak brake fluid. I found the ones that was doing the dirty deed, changed them an voila! No more brake problems. It sound to me this may be your issue. When you bleed them again, take note which ones have a little air. They are the culprits. Anyway, thats my story and I'm sticking with.
Dave

 

Where is the differential pressure switch located ??

What is a DRB ? I don't think I would have one... I don't have a scanner either.

So I "may" have to farm this job out.

??

 

Hmmm..... Very interesting Dave. Thanks for the info.

 

It's in the combination valve. Follow the lines from the master cylinder to where they "split" for the front/rear brakes. There will be an electrical connection to the valve.

A DRB is the Dodge scan tool.

 

You might also have a seized or incorrectly installed brake shoe adjuster on the rear brakes. Pumping the pedal quickly can jack the shoes out to the drum giving a brief high pedal.
Easy to check, and in the old days I've seen people swap the adjusters side to side so they work backwards.
In any event the brake shoes should be close to the drum when you pull the drums off. You'll feel that.

 

I think he said everything mechanical is ok but pulling the back drums off and inspecting everything is a must just to be sure.

 

Will do.

Just for shiitts and giggles today I bled (on the vehicle) the master cylinder. It did puke up a fair amount air bubbles. The dash lights went out and the pedal was "a bit" firmer.

After I put everything back together, the pedal sank back down and the dash lights reappeared as well. That is the ABS and Brake lights BTW.

I'm pondering replacing the master cylinder at this point. I'm thinking the internal seals "may" be blown and fluid is seeping by the piston.

I did take the truck out on some back roads and locked up the brakes a few times. I've read that is "supposed" to activate the ABS and displace air bubbles. It made no difference in the pedal what-so-ever.

??

 

Are you in a rust state if so check your brake lines for bad rust on any of the lines you may have a pin hole leak. Finding brake line leaks can be very hard to find as the DOT 3 almost evaporates and blends in so easy.


Are you losing any DOT 3?


Pull out the brightest flashlight you have and inspect every inch of brake line feel the line with your fingers for oily spots.


Check those back brakes very closely you may want to pull the shoes out and inspect the cylinders for leaks pull the boot back and look for a wet boot.