Very likely your brake master is failing.
1) Check fluid level. Read what is embossed on brake fluid reservoir, on how to.
2) Check for leaks.
As a rule. I tell people, if ABS alarm comes on pull over. Alarm is due to brake fluid pressure below minimum. If this is due to brake booster motor failing to run on demand. You will lose brakes. But you description, does sound like this. But know this Brake booster motors can either just stop working for good. Which yo have no brakes. Or they can be intermittent, which brake may seem okay. But at some point, these intermittent booster motor, will not run and brakes fail.
As long as booster motor runs and we have brake fluid, we have brakes.
Provide brake booster motor is running, when ABS alarm is heard and alarms shuts OFF rather quickly. We can usually drive. Again, provide ABS alarms shuts off.
When ABS alarm comes on, on cold start. Meaning, vehicle has sat without pumping brake pedal, Like when parked over night. Then as IG key turned to ON, brake booster motor runs Until brake fluid pressure reaches, a specific pressure range. If range, not reach or takes to long. We get ABS alarm.
When seal within brake master system weak. They tend to fail to hold full pressure, when cold. As brakes used, fluid heats. Heat expands seal, and thus they seal better. ABS alarms go away. The colder the out side air (OAT). The more this well happen. In time, alarm well become more common. Even on warm days or even after brake fluid warmed up on cold days.
One of the seal that fails, the most. Is mostly due to poor fluid care/health over the years. Black fluid and or stained reservoir are clues of poor fluid care. A seal in front of master, seems to fail most in these. We'll hear a squeal (dying Seagull) and see bubbles come up front of reservoir at same time. The condition just gets worst with time/use.
Also if brake master has a leak. We also get this alarms. They tend to be less reactive to being cold, but they too happen more when cold, at first.
Look a brake master, for leaks. Feel around bottom for any wetness. Especial between ABS unit (black plastic box on side of master, 3 wire housing blocks attach to) and master (silver reservoir and ABS unit attached to).
I have seen, where the ABS unit itself has internal issue. Where only issues is the ABS alarm. We have no other clues. Replacing brake master is the cure.
BTW: The time test of brake booster motor run time. Is after pump brake pedal 40 times, with IG key OFF (Full Evacuated of Accumulator into brake reservoir). We testing how long system takes to build pressure, form this full accumulator evacuated. With 12.5 Volts at battery. We're looking for 30 to 40 sec max. This test is a check for air in system. It also give clues to health of: fluid, booster motor, booster pump, accumulator and seals.
First and foremost: Brakes should only be worked by those, with high level of mechanical abilities and experienced with brake systems.
With the age of all 100 series today, in our aging fleet. Any issue, with brake master assembly. IMHO, best and safest course of action. Have the whole brake master assembly, replaced with new. If you have to ask question, on "how to", your not qualified to work on the master.
My hope is below link will give better understanding as to what we can do to reduce brake failure and what to watch for. The info is from my experiences and my thinking, as to what...
