Weird problem

[MopaR&D]

I have a 1970 Duster with front 4-piston disk brakes and rear 10" drums, and since I got the car the brake pedal would move about 3" with little resistance, then hit a hard spot and the brakes would start to engage. I noticed only the fronts would lock up, so just over this weekend I replaced a wheel cylinder in the back and bled both sides in the back, and the pedal is a bit firmer now. BUT, it only firms up nicely if I push the pedal down, let up, then quickly push down on it again. When I do that the pedal gets noticeably firmer and doesn't go as far, and it feels like my back brakes are engaging. Does anybody know what could be going on? After they firm up and I let off for a couple seconds, it gets soft again and I have to repeat the process. I'm thinking that it's the master cylinder, because that's the only part that's left that I haven't looked at (it's also the incorrect one, it has the "clamp" that comes over the top to hold the cap on instead of the bolt through the center).

 

[mguner]

It does sound like your master cylinder is bypassing internally and possibly even leaking into the booster or down your firewall... take a look.

 

[FASTBACK340]

I agree with Mguner.... sounds like a master cylinder issue. But first of all, do you have a disc brake master cylinder in the car to begin with? If it feels like the brakes are dragging you might have a drum brake master, which is different, and will cause the dragging you've mentioned.

 

[grumpuscreature]

Obvious question, but are the reservoirs in the master cylinder full? Sounds like the front reservoir is low and it takes a couple pumps to fill up and apply the rear brakes. If it's low, fill it up and see what happens. If that takes care of it, keep a good eye on the fluid level in the master cylinder. If you have to add more fluid in a couple of weeks, or the reservoirs are full on inspection, it's time for a new master cylinder. Just went through this with Hester. Hope this helps.

 

[MopaR&D]

I do keep an eye on the reservoir, and that does seem to leak down a bit. I do have some rust on the firewall that must be from the leaking brake fluid. Aw hell, looks like I have a project in my near future!

BTW it must be a disk brake MC, because the brakes don't drag at all. The pedal just acts funny and it leaks a bit.