VenturaToyota

Over the years I have spent hours browsing this forum for answers to small issues with my little pickup. I usually have found great answers and for that I thank you all. I now have a strange issue that I need to ask for a little input on. The truck is a 1988 Toyota Base pickup with 160k. It has a 22R, 4 speed manual, is in good general running condition and has no obvious issues or modifications. I noticed today at a light with the truck in neutral that when I pumped the breaks, the engine stumbles and backfires a bit. I am pretty mechanically inclined and have cared for my own vehicles for years. I have no idea how these two systems can be tied. I am assuming it has to do with vacuum? If you need more info to answer the question pleas let me know. Thank you in advance for any input

Shady Cadence

The brake booster uses vacuum and atmospheric pressure to assist braking. You have a canister, the booster, mounted behind the master cylinder. This has a hose going to it to supply vacuum. Inside the booster is a diaphragm that sits in the middle, with a valve in the center of it, and there's another valve in the back where the brake pedal rod goes in. When off the brakes, the inner valve is open, allowing vacuum on both sides of the diaphragm, the outer valve is closed, containing the vacuum.
When you step on the brake, the inner valve closes, isolating vacuum in the front half, the outer valve opens, allowing atmospheric pressure to push the diaphragm, thus giving you power assist. Is this your issue? MAYBE.

when you hold the brakes, does it stumble? Can you hear a hissing from the dash when holding the brakes?
If it only stumbles when you PUMP the brakes, the booster is NOT your issue. Each time you pump the brakes, you let extra air into the engine. If this is causing a stumble, then your engine is running lean, the extra air is causing a lean misfire. If you were running rich, it would flare the rpm.

Odin

Try a couple of easy tests.

An hour after you shut the engine off remove the brake hose between the check valve and the booster.
You should hear air rushing in if the system is tight and the check valve is functioning correctly= no vacuum leak at that point.

With the engine OFF pump the brake pedal a couple of times and hold the pedal down.
While you're holding the pedal down start the engine.

Does the pedal stay at the same height or does it sink down? If it sinks down your brake booster is functioning normal, if not it's bad and needs to be replaced.