Brakes Seizing After Master Cylinder and Brake Booster Replacement

[WUFFIAT78]

Hello all,

I recently changed the brake booster and master cylinder in my '78. After replacement I bled all the brakes to ensure the air is out. Upon driving everything seemed good. Braking was much improved. After driving for 15 minutes or so, the brakes started seizing and would not let loose. Over several hours of sitting the pressure eventually reduced.

Thoughts?

[18Fiatsandcounting]

This seems to be happening more frequently these days. Apparently there are slight differences in replacement master cylinders and/or boosters in terms of the distance from the tip of the plunger rod in the booster and the recessed cup in the end of the master cylinder. There should be a small gap between those two when the brakes are not being engaged, but different manufacturing tolerances for replacement parts have caused issues with the brakes failing to fully release.

The solution is to adjust the acorn nut on the end of the plunger rod so that there is some clearance. Just a millimeter or two. You can either carefully measure to determine how far to screw down on the acorn nut, or you can use a marking compound to tell when the two components meet, and then back off a bit. Colored grease or the like, and I've heard lipstick even works.

-Bryan

[WUFFIAT78]

Thanks Bryan,

I just called Vick and they immediately stated the same issue. It also looks like I will need to remove the set screw as currently I am as far in as the set screw/acorn will allow. He suggested that once the distance is determined to "glue" the acorn in place.

Appreciate your quick response. Hopefully this weekend I can get it set.

Dwayne

[18Fiatsandcounting]

An easy way to gain more clearance on that acorn nut is to insert spacers on the studs that secure the master cylinder to the brake booster. If you have U-shaped spacers, you don't even have to remove the master cylinder from the studs. Just loosen the two nuts several turns, slide the MC forward enough until you get enough clearance for the spacers, then slide them in. Retighten the nuts and you're done.

Not ideal but it works. I don't think I agree with gluing the acorn nut. Isn't there a locknut of sorts on it already?

-Bryan

[WUFFIAT78]

The reason he stated that is that I currently have the lock nut down as far as possible and will be forced to remove the lock nut all together. Planning on working on it tomorrow. Hopefully its not to much trial and error on the correct setup.

[Mheinle]

I just went through this process. Instead of glue drop a little locktight on it. This will keep it from moving by itself yet allow you to make adjustments in the future. The guy at Vicks said they run this way often. And yes I too had to remove the lock nut to shorten it enough.

[WUFFIAT78]

This did the trick. No issues. With the new brake booster the car is running the best it has ever run, and the brakes are solid. Thanks for everyone's help.

Have a great Thanksgiving.