Hard brake pedal on cold morning first start only - what is the final answer ?
 

I am having the issue of hard brake pedal on first starts only on cold mornings only. Once it starts no issues in braking. By cold I mean not freezing or anything but about 40-45F. Never happens in summer. but longer the car is not operated (12+hours) the more possibility of it happening.
I had the brake fluid replaced about a year ago and I have replaced the check valve as well, (although not the hose from the check valve to the master cylinder - couldn't get to it easily/quickly). And this issue is still happening.
I searched found some threads but there is nothing conclusive. Some say replace brake fluid, replace check valve but I have done those.
Has a certain known cause of this issue determined yet ?

If the pedal is stiff on cold starts, I would check the brake booster.

And I suppose the only fix would be to replace it ?
Is there any other repair or fluid type that would help fix the issue ?

Well, if it's an internal leak in the seals, yes replacement. If it's a leak in the one way check valve outside the booster, then no - check the valve first by blowing through it and ensuring it only allows flow one way. If it's internal or the leak gets larger, you should hear a hissing noise under the dash.

when is the last time you did a brake fluid change? It could be excessive moisture in the fluid causing the issue.

In his original post he said about a year ago.

ahh my mistake, should have read better! I did a brake fluid flush a few weeks ago on my car and when it got down to 35F and more than 12-hrs staying parked, no hard brake pedal (used to have it before the fluid flush) when it was colder out. I used DOT4 fluid as it holds up to heat better.

Which fluid did you use ? When I changed the fluid last year I put in Valvoline DOT4 LV. I am wondering if a higher viscosity fluid might work better. I am going to try Honda oem fluid or a less viscous dot 4 if I can find.

Just to clarify, when you cold start with brake held, does the pedal immediately push down normally? Or you have to drive it a bit to get it normal?

It immediately pushes down normally and brakes normally as well while driving.
btw I have also replaced the rubber seal/gasket inside the fluid reservoir cap. and also that check-valve thats in between the master cylinder and manifold.
It would be great to find if there are any brake fluids that contain special conditioners for internal seals and gaskets in the master cylinder. No leaks anywhere though that I can see.

So in this thread I didn’t understand whether the brakes were gone or not, it feels like you’re writing about a problem that doesn’t exist.
I know many cases where a customer would come up with problems, and another garage would install aftermarket parts, which would really make the car worse, and everything would be resolved when OEM parts were installed back.

Braking while driving was never a problem. I made sure to mention that in the first post/line itself.
The issue is surprisingly quite common for this gen cars but I havent been able to find a specific fix for it yet thats why I created this thread.
so to recap - the issue is that when you first start the car on a cold morning the brake pedal is stiff and does not press down like it normally should. As soon as the car starts the brake pedal works normally again. Its as if it looses vacuum while sitting overnight especially on cold nights.

So when you first go to start the car, and press the brake pedal to enable to car to start, the pedal is hard, but as soon as the car starts the pedal feel is normal? It's been 3 years since I no longer have my TL, but as best as I can recall that's perfectly normal. I don't understand the concern if the pedal feels fine therafter.

In this case, there are only three suspects: the valve, the hose, and the booster. If the hose isn't cut or cracked, and the valve is replaced, then the problem is with the booster. Brake fluid and the brake master cylinder have no effect on pedal feel when using vacuum.