I installed Sumitomo MK63 Calipers and vented rotors on my 72 Z recently. I have experienced a hard brake pedal since that installation. I tested the vacuum assist and I believe it is working. I pumped the brakes, held pressure and started the car and the pedal depressed slightly. I plan to put a vacuum gauge on it tomorrow and see if I have at least 18" vacuum. I believe the next possibility is the brake valve (Pressure Differential Switch). It may have moved off center during the installation and bleeding. I am looking for a NOS brake switch / valve just in case. Any leads out there for this part or any other NOS brake valves or proportioning valves for a 72?    

Edited November 26, 20177 yr by JLPurcell

I thought that the "hard pedal" or abrupt brake actuation was almost always the reaction disc in the brake booster.

I've seen people talk about resetting those switches but I think that the FSM describes that they automatically recenter when the pressure differential problem is removed.  In the later FSM's anyway.  They don't mention any need to replace it in 1972.  Some people think that they divert pressure to the working half of the system but I don't think so.  The master cylinder's serial piston setup will still pressurize just with more travel, I think.  But if you have a hard high pedal that would mean both front and back are getting pressure.

p.s. I think that you're blending up your terms and words.  There's a switch, which you show, and distribution fitting (3 way connector), and a proportioning valve.

1 hour ago, Zed Head said:

I thought that the "hard pedal" or abrupt brake actuation was almost always the reaction disc in the brake booster.

I've seen people talk about resetting those switches but I think that the FSM describes that they automatically recenter when the pressure differential problem is removed.  In the later FSM's anyway.  They don't mention any need to replace it in 1972.  Some people think that they divert pressure to the working half of the system but I don't think so.  The master cylinder's serial piston setup will still pressurize just with more travel, I think.  But if you have a hard high pedal that would mean both front and back are getting pressure.

p.s. I think that you're blending up your terms and words.  There's a switch, which you show, and distribution fitting (3 way connector), and a proportioning valve.

I am a little confused, on "The master cylinder's serial piston setup will still pressurize just with more travel, I think". From all that I can find on this subject, if it is not the vacuum assist then it is more than likely the proportioning valve. I have to say that I have not found any direct information on the 72 Z's system, but on brake systems with a hard brake pedal in general. It made since to me, due to having installed the new calipers. In further reading simply bleeding the rear brakes may center the proportioning valve. I believe that it is #10 in the illustration. I was obviously mistaken in that I thought that there was a proportioning valve in the pressure differential switch as well. In the race car with MK63's the brake system was re-plumed with adjustable proportioning valves when built. I will make sure I still have the proper vacuum and then bleed the brakes starting with the rears and see if that resolves the problem. Thanks for the information. JLP 

Can't help on the hard pedal as when it happened to me it was the disk that dropped in the booster, #10 the rear proportioning valve is fixed, no moving parts that I remember. #8 the warning light switch, the switch itself is the plunger type that threads into the top of the unit. If the 2 ended piston that activates the switch is moved due to loss of pressure in either the front or rear lines it has to be manually reset to center by unscrewing the switch and moving the piston back with a small screw driver or dental pick.

2 hours ago, grannyknot said:

Can't help on the hard pedal as when it happened to me it was the disk that dropped in the booster, #10 the rear proportioning valve is fixed, no moving parts that I remember. #8 the warning light switch, the switch itself is the plunger type that threads into the top of the unit. If the 2 ended piston that activates the switch is moved due to loss of pressure in either the front or rear lines it has to be manually reset to center by unscrewing the switch and moving the piston back with a small screw driver or dental pick.

Thanks for sharing your knowledge on this. That was my first thought after reading up on the subject, but then got a little off track. I appreciate this site, for its wealth of knowledge shared among the enthusiast. JLP

I've kept several of those brake light switches if you want to try a couple of others. Let me know at z240@shaw.ca

zKars, Definitely would like to pick up one of two. I have the habit of looking for extra NOS or good used parts when I have a problem and find that I do not have a replacement part. That is how you end up with a garage attic full of spare parts!

Edited November 26, 20177 yr by JLPurcell

Just now, zKars said:

I've kept several of those brake light switches if you want to try a couple of others. Let me know at z240@shaw.ca

Definitely would like to pick up one of two. I have the habit of looking for extra NOS or good used parts when I have a problem and find that I do not have a replacement part. That is how you end up with a garage attic full of spare parts!

1 minute ago, JLPurcell said:

Definitely would like to pick up one of two. I have the habit of looking for extra NOS or good used parts when I have a problem and find that I do not have a replacement part. That is how you end up with a garage attic full of spare parts!

Well then let's move some of my garage full of spares into yours!

Just now, zKars said:

Well then let's move some of my garage full of spares into yours!

Sounds like a plan! I am working on organizing the parts today so I can get all of my toys in the garage for winter and still work of them!

4 hours ago, grannyknot said:

If the 2 ended piston that activates the switch is moved due to loss of pressure in either the front or rear lines it has to be manually reset to center by unscrewing the switch and moving the piston back with a small screw driver or dental pick.

I think that this might be an urban legend.  Maybe country too.  Just saying, the later FSM's say otherwise and I had the issue on my 76 and it reset just fine.  On top of that, add that it doesn't really make sense to have to reset the switch, since it moves by pressure differential, and it would be surprising if they didn't include this piece of knowledge in the FSM, since they talk about all of the other details.  I haven't seen a case where anybody actually had to reset the switch.  I think that the light just goes out when you fix the pressure differential.  And, to keep piling on, the FSM does say explicitly that the switch should not be disassembled, but replaced. 

Beside that, I don't see that the brake light is staying on in this case, which is the real clue.  You can also check continuity at the switch itself if you're not sure about the wiring.

Anyway, just one of those things that bug me. Carry on.

It might be that the early switches just got gummed up and stuck.  But it doesn't seem to be a factory described reset procedure.

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