Brake pedal to master cyl. Flex

Started by jtb342, November 18, 2023, 10:58 PM

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I'm helping out with an issue after replacing the booster on a 1972 M300 Tradesman. The front brakes are slowly locking up and will release overnight or by opening a front slave cylinder. I can see that there is a lot of flex in the odd system that was engineered to fit in the front engine compartment in order to fit the master and booster ahead of the brake pedal assembly. The bell crank system needs more support to stay stable, and I wondered if this is the cause of the lockup issue. I plan on the support as just a precaution, but wanted to hear from the experts, so I don't do it for the wrong reason.
Thanks in advance,


I'm thinking about clogged rubber lines. did you replace those too?


I did not, and although they look ok, I realize its a problem area and will be changing them. Also going to go through all 4 corners to make sure the brakes are all proper. I was just concerned with the flex in the 90 degree bell crank system, and wanted advice on that, since I haven't seen much on it. I think I will make up a support to test its effect and possibly a upgrade? Thanks for the suggestion though. I also looked at the metal lines, and they are all good to go.


You replaced the booster, but not the master cylinder?? What I am thinking is the master cylinder is not returning all the way back in the neutral position to allow the fluid to return to the reservoir..You press the brake down and fluid goes to the brakes but because the master cylinder and the booster are too tight to each other, the master cylinder isn't returning fully..
There is suppose to be a little play between the booster and MC..This allows the return port of the MC to open..If it is too tight then the port remains closed..Locking the brakes..
The best mechanic is the one that can make it run with the least amount of parts!


The adjustment was correct, I thought, but will check to make sure the master is returned then try it again. The master was changed before the booster, I just assumed it was working proper. I will double check it and make sure. Thanks for the advice.


You don't need to remove the MC from the booster..Just loosen(to test..not a fix) the MC from the booster by maybe one turn of the mounting bolts..This will add clearance between the MC and the booster..Then see if the brakes loosen up..

MLW had good advice on the rubber brake lines..The rubber breaks down and collapses..plugging everything up..

As for the flex..Are you good at welding?? maybe adding some extra steel to a weak area..Just got to add it where it needs it..Even if this is not the cause of the locked brakes, that flex needs to be fixed..

The best mechanic is the one that can make it run with the least amount of parts!


Thanks for the advice Mike. I had the chance to adjust the master clearance while test driving and trying to find a sweet spot to the adjustment. No dice.
Went down today for the first time in a while, so it's time to get busy and fix it proper.
I was bleeding the rear brakes and the passenger side went good, fluid looked good, then the drivers side exposed the real problem. It actually looked good when static testing and when driving it didn't heat up, much, but the fluid really looked bad. It's getting torn into in the next day or so. Planning on replacing the slave cylinders and both rear rubber brake lines to start. The flexing will be done, but I want to wait until the brakes are solid before fixing that. I am not a welder, but not bad at fabricating. Besides, the area in question is expert welder tight, so....
Thanks for all the help everyone. It seems like I get more of these older motorhomes to work on, and although I really would rather do cars, the people that own these, are really my kind of people, easygoing, hard working, real and not too sane,  :)