Working on a friends car - a1968 Rambler American. Has a front disc brake conversion off a 1974 Gremlin. Took it for a test drive, and the brakes are ridiculously touchy. Very hard to modulate with your foot. The go from nothing to locked up within an 1/8 th of an inch of pedal travel.

Owner says this started after the master cylinder was changed some time ago. Brakes were fine before that. A brake shop just replaced master cylinder again, last week. The touchiness of the brakes coincided with the first master cylinder replacement, and remains unchanged after the second master cylinder was installed.

What do you guys think? Bad booster? Issue with master cylinder actuator rod adjustment/length? I may just throw in the towel and tow it to WaJim.

This isn't any help, but my best friend in HS's older sister had exactly the same problem with her Plymouth, about the same year. They were so touchy she was the only one who could drive it, anyone else locked up the brakes every time they touched them. I guess that kept my friend from borrowing her car. She'd had them worked on several times, but the problem stayed, she just learned to live with it.

If a power brake car, inspect the vacuum check valve on the booster. If a manual brake car, inspect the proportioning valve on the frame.

I was going to guess proportioning valve as well


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"...Brakes are good; tires are fair..."

This system has an aftermarket adjustable Wilwood proportioning valve. I turned it as far as it would go in the ”less brake” direction, and that helped some. But, the Brakes are still too touchy

Check valve is new. Doesn’t mean it works, but it was just replaced

disconnect vacuum to booster,( plug of course) and check brakes around the block. if it seems normal id next check the rod between booster and MC and see if its adjustable

the booster that was installed was it new or used, not that it really matters but ive seen boosters go bad and do this very thing

It could be several things.

I don't think the booster is the problem. Even a dual diaphragm booster with discs is not that touchy.

A booster is just a big rubber diaphragm that helps pull in the pedal. The larger diameter the booster the more surface area for vacuum to help "pull" the pedal. When boosters fail they either stop working all together of they leak around the shaft "roll" in the rubber when the pedal is pushed and usually cause a vacuum leak. This usually results in a change in idle speed/sound when the brakes are pressed in and out while stopped.

There is no way a failed booster can create MORE vacuum thus making it more powerful, only the engine can make more vacuum. There is no way a failed booster can gain more surface area and become more powerful. Thats not physically possible.

Failed boosters just make the pedal harder to push or cause a vacuum leak.

Im going to assume the front brakes are the ones at fault since you did not mention it locking up/dragging the rears?

Proportioning valves only limit the braking force to the rear brakes. Front brakes should get full pressure from the master then you cut down the rears until they match the fronts. Since the rear is lightest they tend to be more powerful than the fronts without having a prop valve and being properly adjustment.


IS the master a dual or single?

The volume is also a big consideration. Was it changed with the disc swap or is it the stock drum brake master?

Might have the wrong master cylinder. If the conversion was from drum brakes to disk the master cylinder might have been changed. If somebody put on a new master cylinder recently, they might have put the wrong one on. This is common when people build custom street rods and nobody knows what changes were made or what cars the parts came from.

The booster looks old. Don’t think it has been changed in a long time. The master is new.

I will try the vacuum line disconnect next week, when the rubber brake lines arrive and are replaced.

The master is a dual

I don’t know if the master or booster was changed when the disc conversion was done. All I know is what is there now. I will see if I can find some part numbers. Checking Napa online, the booster on there does not exactly match what is called out for either a 1968 American or a 1974 Gremlin.

I will pull the master and see if the rod is adjustable and whether it is adjusted correctly.

It’s a cluster at this point

Conversion from 1974 Gremlin....



First was the Rambler a Drum brake Booster car?

If it was then its Mandatory that you use Gremlin booster and master. The rod between the diaphragm and master can be wrong length ...while this wont make the brakes more touchy it will be closer to the top of the pedal stroke and may also be dragging the brakes.

If it was Manual Brake?


Remember, AMC Cars used a mix of Ford and Chrysler parts.... there was also time in the 70's that Chrysler had VERY touchy power brakes in the larger sedans but mid 70's AMC was not an issue....

SO....

This is also important. The point on the swing pedal where the rod comes from the brake pedal to the booster or master on a FORD is different from manual brakes and Vac Assist. Some manufacturers used a different location in the pedal hanger to install a longer pedal for Manual brake...so you'll have to do some investigating. Either design the distance from the pedal pivot to the brake rod is less with power brakes than manual brakes.

Remember American Motors is AMERICAN motors....they could be using the same design as Ford when it comes to rod placement and fulcrum point on the pedal.

Also, The master cylinder Has to be from the Gremlin.....



Here is a good article....http://knowhow.napaonline.com/how-to-me ... dal-ratio/

Great questions I have no answers to. No known history on this car. The booster on it does not look exactly like what Napa shows for a gremlin. It also doesn’t look like the booster Napa shows for a stock 1968 Rambler American with front drums. The master looks similar to a gremlin master, but I’m still looking for part numbers on the current booster and master.

Looking at the plumbing on this dual master, the large chamber on the master is plumbed to the front brakes as it should be. However, the proportions valve is on that line, not the rear brake line. That seems odd to me.

Any reason the proportioning valve should be plumbed into the line coming from the large chamber of the master???