Adjusting a Hydroboost pushrod

[lgordon]

I’m installing a replacement Hydroboost unit in my MK4 and need help on how to adjust the Hydroboost-master cylinder pushrod. I know there should be 0.020” free play but in order to set that I need to know how to measure what the length of the adjustable pushrod should be without free play.

I have Mike Forte’s Hydroboost system. Thanks for any help.

[edwardb]

Multiple videos show a tool used to measure. There are multiple versions of this. https://youtu.be/YDlz7-X3S2g. I actually made my own using the same concept. Some guys figure out how to measure with a micrometer only.

[lgordon]

I saw those but also saw that many of them are listed as being specific to certain make, model and year cars. Presumably that’s because the diameter of the cup (and ball end pushrod) in the master cylinder and/or brake booster varies between make, model and vehicle year. I’m leery of using a measuring tool that could give a false measurement.

[edwardb]

I saw those but also saw that many of them are listed as being specific to certain make, model and year cars. Presumably that’s because the diameter of the cup (and ball end pushrod) in the master cylinder and/or brake booster varies between make, model and vehicle year. I’m leery of using a measuring tool that could give a false measurement.

I wasn't promoting that particular tool or any other. You asked how to measure and that technique is one way. Granted that was a vacuum booster but the requirement for a certain amount of free play is common between vacuum and hydroboost. When I did hydroboost a couple builds ago, I was able to determine the proper length with careful measuring using a micrometer. For my current truck build with vacuum boost, I made a tool as I mentioned before. Crude but it worked. I'd be careful using someone else's measurement if that's what you have in mind. You need to measure your specific parts.

[CraigS]

Most dial calipers can be used as a depth caliper by setting the other end of the main body on the edge of whatever and opening the caliper so the rod protrudes from that other end. Mine is perfect. You can check yours by setting it on a flat surface and trying to open the caliper. If it's off a couple thou rotate the dial a little. I don't have a digital caliper so don't know how to zero them but there must be a way.

[lgordon]

Thanks for the suggestions! I do have a digital caliper and a digital depth gauge. Both can be zeroed and are claimed to be accurate to 0.0005”but since neither one has a ball end anywhere near the diameter of the ball end on the pushrod my measurements are still approximations. I’m sure I’m in the ball park. My plan is to pull the master cylinder out of the car so I can do fine adjustments on a workbench. I’ll make small adjustments until I can feel where the added resistance from the master cylinder return spring kicks in and then back off +/- 0.020

[lgordon]

My plan above worked perfectly! Using a depth gauge did get me in the ballpark of the distance between the master cylinder and hydroboost pistons. A few quick trial fits and adjustments and much to my surprise I was able to determine the pushrod length that connects both pistons +/- 0.0005”. (Credit for seeing what pushrod length keeps the HB and MC flanges from easily mating flush goes to Mike Forte. I never would have thought I could feel and see that small of a difference on my own.) I then shortened the pushrod 0.020” installed the mated mc/hb and after bleeding everything the brakes work perfectly!