Ok. So I have a 66 Skylark, 2-door hardtop. It has a crate 71 Buick 350. Other than that all original. Just got it on the road a few weeks ago. Was running fine, a few glitches, but nothing I couldn't handle. Took a road trip to Atlanta from Charlotte, no problems. I figured it'd be ok to do a run to St. Louis. Before I left, the brakes started going out. Loss of pressure, had to pump 'em to stop, ect. I bled them and got pressure back. Drum brakes all around. On the way up it started again. I checked for leaks, nothing. Checked the resevoir level, still full. Bled again, same thing, only lost pressure sooner. Definitely a head scratcher. I thought the master cylinder might be shot. I wanted to upgrade to a dual MC anyway, so I went and got one for a 68 nova. (I had already replace the wheels slave cylinders.) My uncle and I installed it. He's been wrenching a long time. Especially the A-bodies. We bled the new MC. Then plugged the hole on the splitter block that originally went to the rear brakes and added a new line extension for the rear up to the new MC. I screwed up a flare that I had to do, but after fixing that NO leaking fluid. Then bled all the lines,(furthest first). The pedal stiffened up as we went. The driver front wheel wouldn't "spray" like the others, it only trickled. Really hard pedal at this point, so I figured: Time for a test drive. Girl friend got out, I got in and no pedal whatsoever! Engine running or off. Looked at the girl and she swears it was hard. No pun intended. We've tried running this down with no luck. Soooo... Could this be: Lack of a proportioning valve? Do I need one in this application? The piston rod from the booster into the MC is not the right length? If so, what is? Is it possible the booster is shot?
There's a lot of variables that we don't know. #1, you say you have no fluid loss when pumping the brakes. If the previous master cylinder was bypassing internally, then that would explain that problem. double-check your wheels cylinders for leaks. Remove the wheels and drums and check the dust seals for brake fluid leaks. Some dust seals flip out, other you'll have to poke with a small screwdriver or a pick. Check your brake lines for any rusty spots where pinholes could be. One little flake of rust could fall off and create a problem #2 different master cylinders require different length pushrods that go between the master cylinder and power booster. The easiest way to measure is to stick a rod ( threaded rod, coat hanger, etc.) into the hole in the power booster, mark where it comes flush with the opening and measure where the mark is to the end of the rod. Do the same with the master cylinder. Add the 2 lengths together and that's how long you need to make the pushrod or order a pushrod. If you make the pushrod too long (even 1/8 inch), it'll make your brakes drag and the brake pedal will be stiff even before you press on it. Measure the depths of the holes like I mentioned, make the rod ( cut the head off a bolt that is close to the size of the hole, use threaded rod, whatever)and then pay attention when bolting the master to the booster. If there's any pressure or resistance when bolting them together, the rod might be too long. Good luck, and let us know if you have any other questions. -Josh
I thought I'd covered most everything. But you 're right. Lot's of variables. I'll pull the tires today and check the slave cylinders. I've checked all the lines on this car and they are fine. (You'd be amazed at how clean this car is underneath, it's a big part of why I bought it.) I pulled the booster last night. At the hole where the vacuum line goes, I sucked and blew in to it and air pulled into it and blew out of where the rod goes into the MC. I think it's shot. My uncle has a new, dual MC with 7" booster gotten from a local speed shop. The local guys refer to this particular one as a "corvette" booster. It's a universal type one wound use when building a street rod. We'll drop it in today and see if it makes a difference.
Well, the good news is the corvette maste and booster will already have the correct length pushrod between them, so that shouldn't be an issue. Also, the Corvette master cylinder has a larger bore diameter. Basically, pushing more fluid, which means less effort to apply the brakes. If you ever decide to go to 4-wheel disc brakes, you won't have to buy another master cylinder. -Josh
The brakes work great now. The MC was the culprit. The diaphram was shot. I do plan on upgrading to discs all around. One day. I don't think this set up is from an actual vette, could be wrong though. We had to lengthen the actuating rod to the back of the booster. We did this by measuring the overall length of the rod coming from the OEM booster. Then we measured the new one, subtracted that number from the old one. Cut the old one down and tapped a connecting sleeve to go between. To make sure nothing backs off we added a couple off nuts to either side of the sleeve and tightened them down. Now all I need to do is block up the gap around the hole in the firewall. Thanks for the advice.
