As you know transmission fluid is or acts like hydraulic fluid. I don’t think it expands much hot. Never really gave it much thought. I imagine you could check it without your vehicle running or warmed up as long as you knew it was full before hand. I was conditioned to do it one way, so that was that. Oil checking was always after cool down for drain back. But usually is done in my case because I live on a hill while getting gas. Even at home once knowing the oil is full you can check on a hill. Doing it in the same spot with the dipstick marked. All things considered there’s wiggle room that’s why there’s an add and full line. We won’t want to start out at the lowest line. Something Ive rarely experienced. Even back when I went extended driving 50K miles a year or more. That’s the interesting part of this thread when a normal phenomenon. Like checking fluids can turn into such a debate.
I'd say a master cylinder could do this more so than the booster. The booster is just a mechanical assist to the master so that you don't have to use two feet on the pedal every time you stop.
Also, ABS units with sticky check valves can cause this as well.
Then you're saying the motor is full cold on the lowest mark of the sticks range. So,... you intentionally short your oil change a full quart? 😬 Yikes!! 😱
Every motor I've ever changed oil in over 64 years of oil changing that level has been at the top when the book amount of oil is put in and level checked by the OEM procedure. Kind'a the point of the top of the range. A double check if you will.
Have any idea how much of a motors oil is not in the pan at peak pump volume? (second stage at red line). Depends on the motor but 2 quarts isn't out of the question. This one quart range got started when pans held 3-1/2 - 4 quarts. With 1.5 - 2 quarts circulating a motor 1 quart low would, could and often did spin a bearing as the pump suction became uncovered. Air is a pour lubricant. That low mark told the dummy operating it he was a tea spoon away for pulling the pin on that grenade.
We have a bit more cushion today with much larger pans, windage trays and such, but we also have more oil in circulation and rings that are not as adept at oil control and some truly strange pan venting systems that will suck a pan dry enough to be trouble pretty quick if not in good repair.
Give an automatic transmission stick a good look (If you can find a trans with a stick). They are clearly marked for both a cold range and a hot range and many like the JATCO's even have a table in the book showing level V temperature by degrees and voltage and mm on the stick. Even my 6L80E is setup that way.
Engine Oil pan sticks are legacy one quart markers to keep Joey Bumpstick's motor in play.
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