Master Cylinder Pushrod Length and Reaction Disc
03-16-2013, 01:08 PM
#1
Advanced
Thread Starter
Master Cylinder Pushrod Length and Reaction Disc
The purpose of this post is to provide some information I've learned about the correct length of the Master Cylinder pushrod that protrudes beyond the Master Cylinder mounting pad on the Power Brake Booster. (I have a 1971 Coupe with a 454 4 speed).
I have been in a full body off restoration for a few years now and finally am driving the car. However, I had an issue when bleeding the brakes and getting a proper brake pedal and feel. I had bled the brakes using every method possible and each time I felt there was no air in the system. Finally, I decided to check the brake master cylinder pushrod to see if it was adjusted correctly. I discarded this idea initially because I didn't have a problem with the brakes before I dismantled the car. When I checked the amount of pushrod protruding from the brake booster and then measured the recess in the master cylinder valve, it was definitely too short. So I did some measuring and unscrewed the adjusting pin (a lot) until it was at the length I thought it should be. I barely had enough threads left to keep the end from falling out of the pushrod. I put it back together and that made a huge difference in how far the brake pedal went down as well as finally proving that I didn't have air in the brake lines. I have been driving the car and the brakes definitely stop the car but they are very sensitive and I just have to touch the brake pedal before they start applying. I searched the internet looking for a specification on the pushrod length and found a gauge that said the length was supposed to be 1.220" BUT THIS IS INCORRECT unless there is a preload on the master cylinder valve that I'm not aware of. During my research I also discovered that there is a rubber REACTION DISC that fits between the brake pushrod and the brake booster diaphragm. The purpose of the reaction disk is to provide brake feel feedback to the driver. So, I checked my brake booster and sure enough the reaction disc was missing. This explains why my pushrod was too short and also explains why my brakes are so sensitive. If you search the internet for "reaction disc" you find information on Nissan 240z's about this very problem. The reaction disc is sometimes stuck on the backside of the brake pushrod and when you remove the pushrod, the rubber disc falls into the front chamber of the brake booster. If you reinstall the brake pushrod without the reaction disc in place, you will have a very low if not all the way to the floor brake pedal and your brakes will be very sensitive. I couldn't find the reaction disc but was able to get one from another brake booster. I have carefully measured and determined that the proper brake pushrod length sticking out of the brake booster master cylinder mounting pad surface is 1.178" using a digital dial caliper. Using this length allows the master cylinder to fit flat against the brake booster mounting surface with no depression of the master cylinder valve. I checked this using two original power brake master cylinders with 1/ 1/8" bore. Of course, I can be a few thousands of an inch off but at least this specification is very close and I feel comfortable that others can use it. I have read some information about how deep the bore is on the master cylinder valve where the brake pushrod fits and that there are short and deep versions depending on manual vs. power brakes. I have two master cylinders and an extra valve, and all of them have a deep recess in the valve.
I hope this information can help someone that runs into the same situation.
I have been in a full body off restoration for a few years now and finally am driving the car. However, I had an issue when bleeding the brakes and getting a proper brake pedal and feel. I had bled the brakes using every method possible and each time I felt there was no air in the system. Finally, I decided to check the brake master cylinder pushrod to see if it was adjusted correctly. I discarded this idea initially because I didn't have a problem with the brakes before I dismantled the car. When I checked the amount of pushrod protruding from the brake booster and then measured the recess in the master cylinder valve, it was definitely too short. So I did some measuring and unscrewed the adjusting pin (a lot) until it was at the length I thought it should be. I barely had enough threads left to keep the end from falling out of the pushrod. I put it back together and that made a huge difference in how far the brake pedal went down as well as finally proving that I didn't have air in the brake lines. I have been driving the car and the brakes definitely stop the car but they are very sensitive and I just have to touch the brake pedal before they start applying. I searched the internet looking for a specification on the pushrod length and found a gauge that said the length was supposed to be 1.220" BUT THIS IS INCORRECT unless there is a preload on the master cylinder valve that I'm not aware of. During my research I also discovered that there is a rubber REACTION DISC that fits between the brake pushrod and the brake booster diaphragm. The purpose of the reaction disk is to provide brake feel feedback to the driver. So, I checked my brake booster and sure enough the reaction disc was missing. This explains why my pushrod was too short and also explains why my brakes are so sensitive. If you search the internet for "reaction disc" you find information on Nissan 240z's about this very problem. The reaction disc is sometimes stuck on the backside of the brake pushrod and when you remove the pushrod, the rubber disc falls into the front chamber of the brake booster. If you reinstall the brake pushrod without the reaction disc in place, you will have a very low if not all the way to the floor brake pedal and your brakes will be very sensitive. I couldn't find the reaction disc but was able to get one from another brake booster. I have carefully measured and determined that the proper brake pushrod length sticking out of the brake booster master cylinder mounting pad surface is 1.178" using a digital dial caliper. Using this length allows the master cylinder to fit flat against the brake booster mounting surface with no depression of the master cylinder valve. I checked this using two original power brake master cylinders with 1/ 1/8" bore. Of course, I can be a few thousands of an inch off but at least this specification is very close and I feel comfortable that others can use it. I have read some information about how deep the bore is on the master cylinder valve where the brake pushrod fits and that there are short and deep versions depending on manual vs. power brakes. I have two master cylinders and an extra valve, and all of them have a deep recess in the valve.
I hope this information can help someone that runs into the same situation.
03-16-2013, 04:06 PM
#2
Safety Car
Good read. One observation; The pushrod depth for the master cylinder has nothing to do with power or manual. The depth changed from the deep bore to the shallow (more like a divot) bore for the '77 model year. Power and manual master cylinders prior to '77 used a deep bore.
03-16-2013, 07:43 PM
#3
Advanced
Thread Starter
Thanks for the information on the short and deep recesses. I had read that there were different ones for manual vs power in a few strings on the internet but since I've never looked at a manual master cylinder, I assumed (yes a bad thing to do) that there was a short version of my model year.