70 C20 Hard to Brake

[CW4B]

Stopping hard one day. Thought maybe vacuum booster? Hear vacuum leak in area around vacuum booster. Shot some starter fluid around area while running, no reaction. Disconnected vacuum line from booster and hear no vacuum leak as before. Anything else I should check before determining vacuum booster is leaking internally? Also, this is a rust bucket presently, including the booster and m/c. Perhaps trying to repair vacuum pump isn’t a better idea than repacing?

[LH Lead-Foot]

Your vacuum leak under the dash is engine vacuum leaking past the atmospheric
control valve. Pushing the pedal, closes an internal valve to apply engine vacuum to make the booster assist in adding pressure to the brake master.

Removing the vacuum line will remove engine vacuum from the booster, so yes, you will have no vacuum leak.

It is time to make a decision on a replacement booster and master cylinder. These are safety items and not to be left guessing if they will work or not, unless you are in a farm field only and not on the streets. Double diaphragm booster of 10 - 11" is normal for excellent assist and the master cylinder if questionable should be replaced together. Master cylinders are very cheap. Just make sure you brush on the bench top bleeding procedure and if lucky, you may not have to bleed the 50 year old lines at the bleeder valves. If not sure, check them as well before making you entire parts list.
Try Rock Auto and the highlighted part number for viewing the list of vehicles this part fits or used on. Usually accurate.

Depending on past service, but front, rear brake components, plus hoses and metal lines are required to complete assessment on the entire brake system.
Other wise, a new master and booster won't help much but to expose other parts near failure.

Just get into the habit of replacing these parts every 50 years or so. I am picky on brakes, backing plate wear behind rear shoes, high temp lubricant and calipers (If Equipped) lubed to float properly, including the pads fitment within the holding frame so they are free to move.

Just plan on a booster and master...that will get you started.
ASE Master Tech since 1978 / Retired

[CW4B]

Thanks for sharing your knowledge. Will replace booster & m/c. Thanks.

[LH Lead-Foot]

One tech tip when removing the old master and booster. There is a push-rod between the booster and master cylinder. Locate it, keep it and re-use it as new parts don't have this part. When inserted into the booster, it touches the back of the master cylinders piston assembly.
Problems;
Most all are adjustable and length matters. Too short, and there will be a lot of travel when applying. Too long and it may hold the master cylinders piston from rearward travel, not allowing the "Fill" port in the master to allow brake fluid in. As brakes are applied, the hole is covered & blocked off, trapping build to be pressurized.

A lot of guys have been on this forum after this work with problems of travel or unable to bleed. You can see the rear hole in the bottom of your new master in both reservoirs. This is gravity feed and fills the system to allow for wear for rear and front both. Very little movement should close this hole and can be seen in action on the "Bench" bleed procedure for the master.

Just a heads up! Best of luck with your project.