low coolant sign constantly

[AriuSen]

Hi,
Wanted to know if someone could figure out this mystery for me.
I will explain the whole story.

2 weeks ago, I turned on my car in the morning to drop it off at the shop for some work. Car was in the garage for a week. When I started the car, left idling for 5 min, and when I start to back up, I notice little coolant on the floor and then I notice low coolant sign on my dash. So I stopped, opened my hood, and I see coolant leaking from the cap.
I turn off the car, open the cap and fill up a little just to go to the shop. Explained to him what happened and left home. Next day, he shows me a picture with a puddle...the whole coolant leaked. Then he found out the heater core was bad. As I wanted to go so bad to laguna seca 2 days later, I told him to just by pass and flush it. He does the by pass, but he didnt flush, instead he just added coolant back in.
As I am driving to the track, after 2 hours in the highway, I see the low coolant sign again...so I stopped and added maybe a small cup of distiled water, and also checked for any leaks but I couldnt find any. The lights turn off and I go back on the highway......everything was perfect...my temperature was never high or anything, and after 4 hours driving, I get low coolant sign again....i stop again, look for leaks, and nothing again, so I just added water again and left the car parked at the track overnight....the next day, I go inspect it, and I see zero leaks and no sign of low coolant....I tracked the whole day and then drove 6 hours home and no sign of low coolant....so I guessed that I had air in the system, and since its a self bleeding system, as air came out, water was getting low, and that everything was good again.

1 week after, I turn on the car to take it to the shop again, and guess what? Low coolant sign and this time, zero leak on the floor or anywhere else....I add coolant and go to the shop, and explained to him the whole story.
We changed the coolant reservoir, some hoses, and the mechanic told me that hes checking pressure before closing the shop, but the weird thing is that when he checks again next morning, the pressure is higher than the night before....thats not suppose to happen right? When he removes the cap next morning, there is pressure there somehow and when he opens the cap, coolant comes out from the cap even though car was stationary all night.

He told me he checked for cracks or water in the engine oil, and nothing....what could be happening to my car????

[RobMCoupe]

I seen coolant filled expansion tank when there are air in coolant system. Mechanic did not mentioned he bleed the system after changing the hoses.

[AriuSen]

Originally Posted by RobMCoupe

I seen coolant filled expansion tank when there are air in coolant system. Mechanic did not mentioned he bleed the system after changing the hoses.

Yea. I told him to bleed this time after replacing the reservoir.
Will also change the level sensor to see

[StickMon]

There's an incantation you need to perform to properly bleed it.
From this thread:

Remove the reservoir cap and loosen the vent screw.
...
Turn the ignition on (but do NOT start the engine).

Turn heater temp knob all the way to hottest, then set fan to slowest speed (defeat the auto mode and AC). This will open the heater core and thermostat.

Press and hold accelerator pedal all the way down for 10 seconds. This will start the electric water pump's "vent" mode. It will turn off and on for short intervals for about the next 8 minutes.

As soon as the vent screw starts to burp coolant tighten it shut. As the pump turns off and on continue to top off the resevior with fresh water.

[The HACK]

The S54 does not use an electric water pump.

[StickMon]

Shows you what I know.
I assumed the incantation activated #4 here.

[AriuSen]

Thanks everyone for the tips.
I will test this weekend.
Replaced few stuff and hopefully it?s fixed. Sending oil sample also to black stone to make sure I don?t have water in the engine.
Btw The HACK you up for ach this weekend? Going to the coffee place and then come back.

[The HACK]

Can't, family in town from Virginia. Maybe I'll sneak out befor sunrise with a motorcycle buddy to Newcombs for b'fast, but I need tombs back home by 9:00am at the latest.