This tool is used to accurately measure the depth of the bore of the brake master cylinder, in order to allow accurate adjustment of the length of the power booster pushrod. If the pushrod is too long it will engage the brakes all the time, causing overheating of the brake linings and poor braking. If its adjusted too short you can get erratic braking action, and not have full braking force available when the pedal is fully depressed. \ For each car the factory specified a length for the booster pushrod to extend beyond the booster housing, but on older cars where factory correct parts are no longer available its not a good idea to trust the factory spec to be correct. The safest method to ensure correct braking is to measure the actual depth of the master cylinder and adjust the booster pushrod to match. You measure the depth of the master cylinder bore by placing the tool against the master cylinder and depressing the center measurement pin into the bore until it bottoms. The pin will stay in play due to the friction fit between the tool and pin – no locking knob is necessary with this design.Then, you place the tool against the power booster and adjust the pushrod until it just barely touches the sliding measurement pin. This is intended to be a very low cost tool for DIY folks to use. There are heavy duty versions of this tool available for sale elsewhere that would be more suited for daily repeated use in professional auto repair shops. But for the average car enthusiast, this tool is more than adequate. I made this tool for myself initially, but once I used it I realized how valuable it was and made it available to the folks on my classic car forum. The people that have used it have all told me how nice it was to use, and have had very successfully brake cylinder swaps due to the use of the tool.This tool is made in the USA – I print each on a 3D printer, square the ends, and cut the plastic center pin specifically for each one. Colors will vary based on what filament I have in the printer at the time, so dont be surprised if you get an orange, blue, white, or even red one! They all work the same of course! This tool has been used successfully on 60’s-80’s era cars including Dodge, Chevy, and AMC models. You will have to determine based on the configuration of your brake booster and master cylinder if this will work for your situation. Free shipping to all destinations covered by the USPS. If you are outside of that please contact me and we’ll figure out how to ship it. International shipping is available via ebay. I’m not Amazon Prime, but I ship fast! I will try to get your item mailed the morning after you order, and shipping times from here in Kansas via USPS have been 3 days to just about everywhere, and only 2 days to the central Midwest. So most people receive their tools in just 4 days! Here’s some of the recent comments that I’ve received on the tool: Pin adjustment gauge worked very well on 1930 model Ford hot rod with booster brakes. It worked great used it on a Foxbody mustang. Was very impressed with the 3D printing. I used it on a 1969 Mustang and on a 1963 Datsun so far. Works good. neat trick to check brake booster rod Cheap tool will work as good as any Excellent product. Super fast shipping Tool came fast and worked great Thanks Great idea and execution!! I love our American ingenuity. Got it 3 days sooner,just what I needed. Thanks Good item it did the job thanks! EXCELLENT. MAN THIS CAME IN HANDY