Brake Servo Questions

Good day, I'm in the process of fitting a new brake servo to my 1975 SIII 109. Single line system out of the brake master cylinder. The brakes were working well, just had a really hard pedal, hissing noise while pressing the pedal, and increase in RPMs when pressing the pedal. All signs a new servo was in order. I've installed the new servo ordered from our hosts (ProLine), and was able to move the master cylinder aside so I didn't have to disconnect any hydraulic line (no bleeding). Some observations:


1. The brakes work with a much longer pedal travel than before, but tAttachment 14404he brake pedal now does not return to the full upright position after pressed.
2. I noticed that the servo pin that engages the brake master cylinder (see pic) is very easily pulled out of the servo. I can't completely remove this pin from the servo, but I can definitely pull it out by hand far enough to where it unseats itself inside the servo. I then have to push it back into the servo and wiggle it around to get it re-seated in its housing. Is this normal? On my old servo, this servo pin on the master cylinder side moves together with the servo clevis rod in the pedal box. They are one/attached. From an engineering perspective, it doesn't seem correct that the servo pin on the master cylinder side can be manually pulled out and unseated. If I was to really press hard on the brakes during an emergency stop, it seems possible that this pin could travel far enough out and unseat itself from inside the servo housing. RN sent me a second ProLine servo, and it's the same. I'm hesitant to adjust the length of this pin as I know that can cause the brakes to completely lock up. My understanding is that adjusting the pin length is not necessary or recommended.

For my own knowledge of learning how these systems are supposed to interact with one another, is it the responsibility of the brake master cylinder to push the servo pin back inside the servo after braking is released? Or is that the job of the servo spring to pull the pin back inside the servo housing? If the latter, I can tell you this is not happening. This ProLine servo is completely relying on the spring in the master cylinder to push the pin back into the servo, which in my mind is incorrect.

I hope this all makes sense. Would love to hear from others' experience. Thank you,
-Ben
Video Here:
https://drive.google.com/file/d/1Dgb...w?usp=drivesdk

Basically, the piston of the booster is supposed to work with the brake pedal. The pedal return spring(s*) is what returns the pushrod when you let off the brakes. The pushrod isn't going to go out far enough to fall out. Here's a good video explaining how to adjust the booster pin/pushrod. I used the clay method, which worked fine. Biggest thing is not to have it too tight.
*Note, the booster pedal towers should have two long internal springs to return the pedal (and booster pushrod).

https://www.youtube.com/watch?v=WI_CXBJ6J3E

Quote:

---

Originally Posted by jp-

Basically, the piston of the booster is supposed to work with the brake pedal. The pedal return spring(s*) is what returns the pushrod when you let off the brakes. The pushrod isn't going to go out far enough to fall out. Here's a good video explaining how to adjust the booster pin/pushrod. I used the clay method, which worked fine. Biggest thing is not to have it too tight.
*Note, the booster pedal towers should have two long internal springs to return the pedal (and booster pushrod).

https://www.youtube.com/watch?v=WI_CXBJ6J3E

---

Thanks JP! Your post is very helpful! I bought the tool in the video to properly adjust the gap between the master cylinder and the servo pin. I'm not sure why others are saying you should never adjust the servo pin? That doesn't make sense to me - then why have the pin be adjustable in the first place. My pin definitely needs to be adjusted.

Follow-on question, my master cylinder looks "newer" (not the original). Fluid looks clean but can never hurt to flush and replace it. It has a slow leak coming from where the plastic reservoir body mates to the master cylinder. In the shop manual, (70.30.01), it describes a bolt that secures the reservoir body to the master cylinder housing. I think my bolt is loose because the reservoir is slightly twistable from side to side. Has anyone had success tightening this bolt, or should the whole unit be replaced? I'm wondering if I can tighten the bolt without draining the reservoir (using very clean tools) - to avoid having to bleed the system.

Option 1: tighten bolt without draining any fluid and no bleeding
Option 2: drain fluid, tighten bolt, replace w/ new fluid/bleed
Option 3: replace MC, new fluid/bleed

Any thoughts? Thanks!

Glad to help.

Yeah, my booster pin was pretty close, but I still had to move it some also. Always better to check it. And as you say, if your master cylinder is not stock, the booster pin location could be way off anyway.

If you can access the bolt to tighten the reservoir, I would try it with clean tools just to see, before buying a replacement. On some of them there are two little screws under the front that hold the reservoir on (not inside the reservoir).

my pedal return spring was broken (missing) for years, kind of a hassel but it came back up at least part ways, sometimes i'd pull it up with my toe but i never had a rod fall out.