In reply to Mazdax605 :
Yeah, sorry about the novel. When troubleshooting I always think it's best to give ALL the information.
I agree having the rear shoes adjusted properly is key to a good pedal but that's not what's going on here. My last attempt involved bleeding the brakes and testing the pedal with the drums locked. I cranked all four shoes out as far as they would go.
I haven't confirmed yet but it doesn't appear the proportioning valve is to blame. I took it apart and nothing seemed obviously out of place. I cleaned everything up real well and put it back together. The system has been bled again multiple times over and the brakes feel the same with the car off. Really, it feels no different than the pedal on my FC. Maybe a little more pedal movement, but again, drum brakes. It really doesn't feel like fluid is bypassing a piston because pumping the brakes does nothing to change the feel nor do I have a slowly-sinking pedal. The only explanation would be a completely failed seal, which I would expect to allow more piston movement on the bench or with the car off. The issue only appears when the car is running but I've found no examples nor can I explain how a booster could be providing TOO MUCH assist.
infernosg said:
I've found no examples nor can I explain how a booster could be providing TOO MUCH assist.
so, there's a valve assembly inside the booster that controls the rate at which the assist is provided. it would be an unusual, but not impossible, failure mode for that valve assembly to do weird things with the assist rate.
learn more than you wanted to know about these components by reading the "description of the prior art" in this Bosch booster patent
In reply to AngryCorvair (Forum Supporter) :
Oh man do I hate reading patents. So much legal jargon and very little technical information. There's anecdotal evidence online that a new booster has fixed a soft brake pedal but everything I know suggests that shouldn't be the cause. I pulled everything off today in hopes I might be able to pull the booster apart but it doesn't look like that's going to be easily done. I'll just sit and wait for the extra parts to come in and try again.
Another thought: this car uses the old "sliding wedge" style calipers up front. I haven't serviced them since owning the car so I'm sure whatever grease that was applied is now long gone. Calipers are supposed to be self-adjusting but if somehow they are stuck and the piston is retracted in the bore I suppose that could cause extra pedal travel. The only time I've touched the calipers is to replace the rubber line going to them. The old lines were a bear to get off and I definitely moved the caliper a little in the process. Maybe that pushed the piston in. Also, the new crush washers I used are slightly thinner than the originals. Same ID and OD, just thinner. Any likelihood these or a slightly twisted rubber line could cause the issue I'm having? Seems unlikely because there's a fair amount of drag on the rotors. I'm desparate at this point.
Bumping this one last time to say I have found the solution! It turns out the problem WAS the booster. Let me explain...
After something like 15 pages of Google search results on RX7Club I came across one thread where the OP was adamant that he fixed his low/soft brake pedal with a new booster. Like me, the replies were skeptical. "A bad booster can only result in a hard/firm pedal," everyone replied. After some back-and-forth the OP linked to a post on AusRotary where someone with a RX2 complained about a similar problem. One commenter mentioned something about a "reaction disk" and that it falling out of place may be the culprit. A Google search for "reaction disk" turned up hundreds of results of 240/80Z guys having similar issues.
The reaction disk is a small, quarter-shaped piece of rubber about 5 mm thick (more on this in a second). It normally sits underneath the booster pushrod in a cup inside the booster. The reaction disk is the thing on the far right of this photo:
This is after I removed the front seal and pushrod from a scrap booster I received. I tried to get a picture of where this thing usually lives. This is looking into the front of the booster with the pushrod removed.
It sits under the pushrod like so:
After I learned on this it immediately dawned on me that I had figured out why my booster pushrod to MC clearance was so far out of spec. I measured it at 5 mm - exactly the thickness of the reaction disk. To confirm I pulled my good booster, was exceptionally careful are removing the front seal and pushrod and peered inside. Sure enough, no reaction disk to be seen. I can only assume my constant turning and flipping the booster around while I cleaned, sanded and painted it caused the pushrod to extend and the disk to fall out of its cup. Shaking the booster revealed a dull rattle one would expect from a small piece of rubber bouncing around. Anyone who's ever dropped a pick inside an acoustic guitar knows what I did next.
I cleaned and re-lubed all the moving parts, put a small dab of epoxy between the disk and the pushrod to ensure this would never happen again, then reassembled the booster. Popped on the MC, re-bled the system for the 20th-odd time, cross my fingers and started the car. I finally had normal brakes again! I don't know how exactly this little piece functions but it definitely has something to do with regulating the amount of assistance the booster provides. So there you have it, the booster can cause a low/soft pedal but it's an extremely unique situation.
In reply to infernosg :
Wish I would have seen this 3 years ago.... good job!
Your perseverance is an inspiration to us all.
Its threads like this where people track down, solve problems, and then explain the answer is what keeps me coming back to Grassroots Motorsports.
Thanks, all. It's such an odd problem due to one having to really mess with the brake booster, which few of us do. I hope that anyone else who runs into a similar problem can find this more quickly than I found a solution.
I did want to make a few final comments. The patent link that AngryCorvair posted correctly identifies the problem part. After reading that for the tenth time (I hate legal-ese) I think I understand how the reaction disk works. It seems like a simple round disk but up close it has three small features: a circumferential bulge that has a diameter slightly larger than the cup in the booster, one or more axial cuts in the bulge, and a smaller circumferential burr that has a slightly smaller diameter than the cup. The bulge seals up against the cup but the three cuts allows some air to bypass it under light braking pressure. This bleeds off some vacuum and results in less assistance. Under hard or panic braking the reaction disk is squished axially causing it to expand radially, which causes the bur to seal up against the cup. This stops all air bypassing the disk resulting in full vacuum assistance. Simple, elegant and (usually) foolproof.
In reply to infernosg :
"you know, i've learned something here today." - K. Brovlofsky
you have educated me with that explanation. I was not aware that, in some designs, the reaction disc could be dislodged. i have always thought it was trapped inside the valve assembly. that's why i said "unusual, but not impossible." so yeah, i appreciate your tenacity. internet high five headed your way.
infernosg said:
Bumping this one last time to say I have found the solution! It turns out the problem WAS the booster. Let me explain...
After something like 15 pages of Google search results on RX7Club I came across one thread where the OP was adamant that he fixed his low/soft brake pedal with a new booster. Like me, the replies were skeptical. "A bad booster can only result in a hard/firm pedal," everyone replied. After some back-and-forth the OP linked to a post on AusRotary where someone with a RX2 complained about a similar problem. One commenter mentioned something about a "reaction disk" and that it falling out of place may be the culprit. A Google search for "reaction disk" turned up hundreds of results of 240/80Z guys having similar issues.
The reaction disk is a small, quarter-shaped piece of rubber about 5 mm thick (more on this in a second). It normally sits underneath the booster pushrod in a cup inside the booster. The reaction disk is the thing on the far right of this photo:
This is after I removed the front seal and pushrod from a scrap booster I received. I tried to get a picture of where this thing usually lives. This is looking into the front of the booster with the pushrod removed.
It sits under the pushrod like so:
After I learned on this it immediately dawned on me that I had figured out why my booster pushrod to MC clearance was so far out of spec. I measured it at 5 mm - exactly the thickness of the reaction disk. To confirm I pulled my good booster, was exceptionally careful are removing the front seal and pushrod and peered inside. Sure enough, no reaction disk to be seen. I can only assume my constant turning and flipping the booster around while I cleaned, sanded and painted it caused the pushrod to extend and the disk to fall out of its cup. Shaking the booster revealed a dull rattle one would expect from a small piece of rubber bouncing around. Anyone who's ever dropped a pick inside an acoustic guitar knows what I did next.
I cleaned and re-lubed all the moving parts, put a small dab of epoxy between the disk and the pushrod to ensure this would never happen again, then reassembled the booster. Popped on the MC, re-bled the system for the 20th-odd time, cross my fingers and started the car. I finally had normal brakes again! I don't know how exactly this little piece functions but it definitely has something to do with regulating the amount of assistance the booster provides. So there you have it, the booster can cause a low/soft pedal but it's an extremely unique situation.
Ha, this is funny that I saw you post these same pic on the RX-7 facebook page, and thought of this thread. Of course not knowing you were the person posting on here. Glad to see you have at least figured out the problem.
In reply to Mazdax605 :
Thanks. I was at my wit's end so I pretty much spammed this everywhere I could.
