Brake pedal sticks. What am I missing?

Ok I’ve been battling this issue and am at my wits end.

1970 skylark, power brake car, low comp 455 w/ stock cam. Recently swapped to a 4 speed. All Bear Brake components, including billet 1 piece master cylinder and adjustable prop valve. 6 years old, little to no miles on it. When I brought the car to my house to tear it part I had no brake drag issue driving it from the tow truck to my house. Upon tearing the car apart I had to remake 1 brake line to clear the z bar and installed a new right-stuff power booster ordered through Summit. Hitting the brakes while the car is running will result in them sticking firm, not to the floor. By pulling the pedal back up with your foot, the brakes disengage allowing you to roll freely.

I thought it was a master cylinder issue, so I called Baer's tech support and they were willing to have it shipped in and rebuilt for free with new seals, but insisted it was a booster issue since they only stuck on with the car running (i.e with it pulling vacuum)... that made logical sense to me. I swapped the old, known working, booster back on that was on the car and it seemed to make the issue less frequent. Called Summit, got a replacement booster and we are back to square one. the right-stuff booster has a threaded rod to adjust the pedal height. I have it sitting exactly where it would with the factory booster.. about even with the accelerator pedal.

Here is what I have done:
-Swapped 2 new boosters and 1 factory GM booster. All 3 have the same problem, but for some reason the GM booster didnt seem to hold it as aggressively.. maybe it has a small vacuum leak to begin with masking the problem all along
- Took the master cylinder off and bench tested it, confirming the pushrod moves in and out freely. Bench bled before installation and assembled correct with previous "bullet" for correct booster rod to master cylinder contact. Also installed in the correct hole on the brake pedal - lower hole
-adjusted to try pedal at different pedal heights. makes no difference, pedal still sticks.

Miscellaneous things that were done to the car during the 4 speed swap that could impact brake system:
-new reproduction clutch and brake pedal setup
-1 new brake line (from a "T" fitting to the front left wheel), all 4 wheels were bled after installation
-new "T" fitting that has a bracket that bolts to the frame. Otherwise the old "T" and new "T" are identical
-new 3.42 8.5 limited slip built by Brian Trick. Only referencing this because I had to swap my rotors, brackets, calipers, hard lines, etc over.

Possible new causes:
Failing brake hose(s)
Master Cylinder is just bad and not showing anything obvious
Another (!) bad Right Stuff booster.. FWIW I installed one of these on my Dad's 72 GS 455 Convert and it has been issue free.
rebleed? could air in the lines cause symptoms like that?

What am I missing here? This was a long way of saying I think I've done everything logically and still cant figure it out. I want to drive this damn car!!!

 

Are you sure the nub that sticks out of the power brake booster is not too long? They have deep and shallow ones.

Did your booster have a long and short pin like this picture?

Also, with the booster bolted on but the master off, does it hang when the engine is producing vacuum? You can pull the master far enough away to not be engaged and leave the lines attached. That might point to the booster or the master.

Next, I would test it all connected but without vacuum on the booster. Does it hang then?

 

Thanks for the suggestions Greg. It did come with 2 rods, currently running the short rod with Baer’s bullet adapter that goes in the back of the master cylinder. Your troubleshooting tips are easy enough to do. I will try them and report back. Maybe swapping the long rod in without the bullet will be better?

 

Could be, like I said I’m not ready to rule anything out. It makes sense to me to try the long rod without the bullet. I’ll report back

 

well, so far a squirt of penetrating oil on the bullet has fixed the problem. It no longer hangs up. Why? No idea. If the problem returns I think I have the solution. I took the long rod with me and compared the two lengths with the bullet. The long rod by itself is just slightly longer, so ruled that out as a solution. However, I noticed the bullet has a witness mark deeper in the recessed area then how the new rod fits in. I measured the new rod to the original and it’s .335” thicker. If the problem returns, I am going to reem the bullet out a little allowing for the new rod to sit in it more flush and hopefully that is the final solution.

sometimes, stupid problems have stupid solutions. I don’t know, this car has put me through the wringer.. can’t wait to encounter the next problem!

 

Nice! Glad you figured it out.

 

Hope all you did solved the issue, but, if it occurs again, check that the bolt that attaches the pushrod to the pedal does not bind, it must swivel or it will stick. Just a thought. jt

 

I know how to solve the problem Louie . Sell me the car for $500 and you'll be ahead of the game .

 

Interesting. Are you referring to the jam nut threaded on the rod? The bracket has a little play in it side to side, but I have the jam nut tight.. The stock booster and bracket were both tight

 

The nut and the through bolt must, of course, be tight, but they should not pinch the pushrod tight to the pedal, it must be able to rotate on the bolt. Think of it like this, if the front end of the pushrod were not supported, it should be free to swing down in the front. jt

 

Ok, sorry, after rereading your post, I'm not sure I'm telling you correctly. Does your pushrod have a "u" shaped bracket that straddles the brake pedal with a pin through it?? If so, it may already be free to pivot. My apologies if I caused confusion. jess

 

No worries Jess, that makes sense. It does have the U bracket and should freely pivot. I am going to reconfirm its not binding up as I am still not sold the penetrating oil solved the problem.

 

FYI
Many times on reproduction boosters I have encountered the 2 booster to master pushrods or pins not being the correct length. Have had to grind it shorter to get it correct length.

As a test use an unmodified pin and place flat washers on the MC mounting studs as spacers. You can try different numbers of washers to get the correct distance then cut the pushrod to match.

 

I would pay double that just in case

 

As we all expected, it did start acting up again. I modified the long rod and took about an 1/8” off. It’s about 90% there, still hangs occasionally. I bought the correct tool off eBay and will have it Friday for final modification. It also had some air in the system. Onto the next problem. Thanks all

 

OK Cork . I guess we are off to the races with the bidding war . I was going to wait till Louie Fixed the problem before I upped the bid . LOL

 

Years ago, I had a Jeep Cherokee that whenever the engine would heat up (hot day with AC on) the brakes would start dragging, getting super hot, and smoking. Long story short, it ended up being the pushrod in the master cylinder expanded when the engine was hot. Expanded just enough to stick. A little sanding to make the rod diameter a tiny bit smaller fixed the problem.

 

Update. .006 clearance between booster rod and back of master cylinder bullet and it still occasionally sticks. Not every time, but enough to be annoying. Might try sanding the bullet down a little to smoothen it out. What a stupid issue