Greetings. New here. We have a 72 246GT The car is in excellent shape, had a full resto in the 2013/14 timeframe. 44,000 miles on it. We also have a 1958 250 GT Berlinetta TDF and a 275B (not sure of the year as it is in restoration) The Dino is new to us and has a braking issue that we have not yet gotten cleared up. The brake pedal is initially stiff, you push through it and it drops just a bit (1/2 to 1 inch-ish), then starts actuating the master cylinder and braking is normal. The problem is; when you push past the initial hard spot and then get the slight drop, then into actual braking .... you can't modulate the initial braking. You're already pushing on the pedal to go past the initial hard spot at the top of the travel .... then dropping into actual master cylinder actuation. We have diagnosed out the pedal linkage. It will still do this with the linkage disconnected, actuating the booster with a lever. When the initial stiff spot is pushed past, a vacuum whoosh is heard, then it goes into normal operation to push the master cylinder. Through this, we have had the booster rebuilt, at Apple Hydraulics in NY. No change. Now, I don't yet know if they just replace the diaphragm or if the valve is worked over as well. My theory, at this point, is that the reaction disc (in the booster) is perhaps hardened and no longer allows the slight travel of the input pushrod and piston/valve, (without actually pushing on the master cylinder) which allows the vacuum port to close, on the input side, and allow atmospheric pressure to fill the input side of the camber .... before actually moving the master cylinder push rod. If the disc were not compressing, this could cause the initial hard pedal, as both sides of the chamber have vacuum, them once enough input pedal pressure is applied to actually move the master cylinder, the piston/valve movement closes the vacuum port and Voooom, you now have boost and all is as it should be .... except for the initial hard pedal and then quick drop .... and this could explain the whoosh that is heard. Thoughts?
Might be inside the booster, seems most likely. Did you disconnect the vacuum hose to the check valve next to the inlet manifold and try it? Check valve could be installed the wrong way so pressing the pedal builds pressure?
The vacuum is fine. Builds and holds ... and, once you push past that little "bump", the booster is working properly and providing assist. Holds vacuum after engine is off, for a few pedal pumps.
I recently had the exact same thing. Started with linkage, then the master all ok. The booster had been rebuilt by Power Booster Exchange who has done many for me. I had another booster that had also been rebuilt and I put that one in and everything worked fine. I sent the one that has a problem to Karps and I explained what the problem was. It should be on its way back to me soon. So I can say your problem is definitely in the booster. I would like to know what was the cause in the booster. I've done over 20 Dinos and this is the first time I encountered this problem.
Omgjon, I talked to the rebuilder at Apple Hydraulics, last night. He didn't have much to say about the actual valve assembly, but said he'd "check his manuals" and get back to me. Yesterday, I went back to it and adjusted the output pushrod all the way short. The idea being that this would give room for valve movement and actuation before actually pushing on the master cylinder input. The "bump" was still there. All this did was give more pedal travel after the "bump", before the pushrod got to the master cylinder. Soooooo.... this leads me to think that the valve assembly is the issue, but perhaps it is hanging up at initial application vs. just an issue with the reaction disc (if this design even uses one ... some just use a gap in the valve mechanism to allow that initial valve actuation). Let me (us) know what the rebuilder noted on the one you currently have out.
By the way, for anyone who may be searching on issues with vacuum boosters, to help with symptom diagnosis, here's a well done "how it works" animation on how the magical vacuum brake booster works. Note that the design shown uses a "reaction disc" to allow valve actuation before actuation on the master cylinder. Other designs use a gap in the valve assembly.
Jon, long time no see....I trust you and the family are well. With regard to the initial "stiffness" of the brake pedal. Of my three Dinos all have had a perceptable "stick" at the initial movement of the pedal. I believe that this is an artifact of overcoming the poppet valve resistance. Not troublesome, but noticable. That said, I am having an issue with one of the Dinos with a brake warning light on. I have bled several times and it is still there. I am about to take out the balance warning switch to clean and rebuild. But my booster will probably come out at the same time for another reason. M3Eater suggests that he has lengthened the actuator rod to maximum. This can be dangerous as the length of the rod defines when the poppet begins to actuate. I believe that there is a critical geometry that must be respected, as is discussed in the Dino workshop manual. M3Eater mentions Apple Hydraulics, while you sent your boosters to Power Booster Exchange, which appears to be more expensive based upon web information. Any thoughts? Jim S.
Note that my lengthening and shortening of the push rod (from the initial 0.5 to 0.7mm clearance) was only to diagnose how this affected the issue that is being tracked (the initial stick at the top of the pedal). It changed how much free play there was, but the initial stick was still there in all settings. Have not yet heard back from the rebuilder, per my discussion with him about the valve assy. Jim, your issue sure seems to be a sticky balance switch. This one had the balance switch valve rebuilt prior to delivery to us, due to the brake waring light being on.
Thanks for the suggestion. I concur. i rebuilt my balance switch about 18 years ago (when Jon came to visit my garage). Likely needs some TLC. Will get under the car and remove it for inspection. Ahhhh, but how I love to bleed the brakes.
If you can adapt a pressure bleeder to the two reservoir filling ports (caps), bleeding is a breeze. Of course, vacuum bleeding is an option, but this sucks air past the bleeder screw threads and you constantly have a stream of bubbles. Removing each bleeder and wrapping the threads with Teflon tape helps ... for a bit.
