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Low brake pedal

Mike Szadaj

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St. Clair Shores, Mi
1968 Coronet born with manual drum brakes. Now with power disc/drum aftermarket setup. Currently using the OEM distribution block( no metering valve) with a separate proportioning valve for the rear. After bleeding the pedal is high and stiff with the engine off, but with engine running the pedal goes very low and the Brake System warning light comes on. The brakes will stop the car, but with effort, and not quickly. The system is 100% new and/or replaced from the pedal to the rear cylinders. I am looking at 3 possible problems. The master cylinder, the booster, the mechanics from the pedal to the booster. How do I check out these components? I am trying to find 2 plugs for the master cylinder to plug the outlets and see how the pedal acts. How do I test the booster?
 
The fact the booster causes a much lower pedal pretty much proves it's working, your problem is elsewhere.... Plugging the ports of the M/C is good, it'll prove the M/C is good or bad... My guess is good.... The light coming on indicates pressure is significantly different from one side of the system to the other.. Have you verified the fluid level is stabile?

Take the car for a slow drive on gravel & try to lock the brakes.. In gravel it's pretty easy... Get out & observe the tire tracks, did all four lock?

With the brake warning light chances are only two locked & now you know which half of the system is causing you trouble...
 
Back brake shoes aren't out far enough.. ie too far away from the drums. Scared myself on the Bird with the exact same symptom and turned out that I had the shoes too far back and the distribution block took the pressure differential as a blown line.. lighting the brake failure light. Shoes brought out closer to drums and all is well !
 
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I thought mine were too, but they weren't. Only took a few clicks on the adjusters to move them out, stop the light and have all 4 wheels lock up in the dirt.
 
The rear drums are snug to the shoes. I wish it was that easy!



I don't understand what you are asking.

Is the brake fluid in the master cylinder full? As I said before the brake warning light coming on means the pressure isn't equal from the front and rear systems so it's possible you have a minor leak...
 
Rear brake shoes drums must be adjusted correctly.

Also, check that the push-rod that actuates the master cylinder is adjusted correctly....I don't remember the actual gap, but it is almost an "interference fit" - ie/ minimal play
 
It is also worth noting that new brake pads up front will give the feel of not working initially....they need to be 'run-in' before they start working correctly.
 
With the rear tires off the ground and trans in neutral, the tires are tough to turn, so I can't see clicking the shoes out any further. The drums would have to be beat on.
The only loss of brake fluid has come from bleeding. I have been underneath this car a hundred times in the past five days, checking for leaks as the bleeding has gone on.
Also, check that the push-rod that actuates the master cylinder is adjusted correctly....I don't remember the actual gap, but it is almost an "interference fit" - ie/ minimal play
That was the third possible problem in the original post. I am going to try to contact MBM to get some insight on that.
 
Is the master cylinder the correct one for the application? Or is this the original master cylinder for drum front brakes. If so, you need to either replace the master cylinder, or modify the port to accept the new disc brakes. This is best done by an experienced person, as it is easy to damage the master cylinder in the process.

Without the correct master cylinder OR modification, you will not be able to bleed the system correctly and therefore have crappy brakes as a result.
 
With the rear tires off the ground and trans in neutral, the tires are tough to turn, so I can't see clicking the shoes out any further. The drums would have to be beat on.
With your brake shoes that tight, your rear brakes are dragging all the time, Mike. You should back off the adjusters, equal number of 'clicks', until the wheels just turn freely.

Agree with what the guys say, about the master...needs to be the correct type, for the brake system. Assume you know, even for disc/drum, there's different masters for boosted, and non-boosted brakes.
 
I am getting the feeling that the MC is not compatible with the disc brakes.


Thats possible but wouldn't explain your low pedal & brake warning light....

Post a picture of your calipers, mounted on the car...
 
Master cylinder. Incorrect piston size. You need to check the pressure at the caliper.
I bought a disc brake conversion from a known vendor. Their tech line was next to useless. They supplied the master.
Only through conversations with Cass at Dr Differential took me to the fix. I went with a master with a smaller piston and that gave me more pressure at the caliper
I also did switch to a dual diaphragm master.
This could be your problem assuming everything else is ok
 
From the Rock Auto newsletter today...a blunder by another motorist in USA...
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I had been planning on replacing all of the brake lines on my Chevy Silverado for some time, but never got around to doing it. But after a weekend of hauling loads of wood, one of the rear brake lines blew out and forced me to take care of the delayed repair. Since I would be doing all the work anyway, I also decided to replace the front calipers.

I was rushing through the job and not paying much attention. Once I routed all the new lines and put on the new calipers, it was time to bleed everything. But during the bleeding procedure, I noticed I had a ton of fluid pressure in the rear calipers and almost none in the front. Also, the pedal was not getting firm. After about an hour of scratching my head and much colorful language, I discovered that I had routed the brake lines through the proportioning valve incorrectly. But, my story does not end there.

I rerouted the brake lines to the valve and tried bleeding the brakes again. My father and I bled the brakes over and over and over, but the pedal was just not getting firm. I had bled brakes before, but never have I had to bleed them as much as I was. I looked on forums, asked friends who had similar trucks and looked at my brake line routing again; nothing worked. I eventually surmised that it might be the ABS module needing an electronic scan tool to properly bleed. I did not have that type of tool, so I threw in the towel and took the truck to a nearby shop.

The next day I got a call from the mechanic...I could not believe what I had done. I had put the calipers on upside down and on the wrong sides! With the bleeder screws at the bottom of the calipers, no amount of bleeding would have removed the air bubbles from the brake fluid. I had to pay the mechanic $240 to fix my blunder! Do not make the same mistake that I did. Take your time and do not rush a repair.

Cody in New Hampshire
 
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If the MC is not providing enough fluid to the front brakes, but it is to the rears, wouldn't that cause the warning light to come on?
 
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