Some brake master are very hard to diagnose, even for me when I've hands on the vehicle. It sometimes boils down to, clues of what it's not.
The fact low voltage of failing alternator. Resulted in, ABS alarms being more frequent. Indicates weak brake booster system. Meaning it's, having difficulty keeping pressure above minimum safe level, within allotted time frame.
One or more of below, are the most common cause:
- High resistance in circuit to brake booster motor. Slowing motor RPMs.
- Weak motor.
- Week pump.
- Weak Accumulator.
- Master ASSY, has leak. Which may or may not be visible externally.
The 35 seconds to fully pressurize a fully evacuated accumulator. Does make me think, good voltage to motor. Since your dealing with a very old master ASSY, that's not bad. But still, did you check voltage at battery post and then down stream. To make sure, no voltage drop after battery. Which a drop, indicates, increase resistance down stream of battery. Something like loose connection, oxidation on battery clamps or wires leads, oxidation inside wires with sheathing or even inside fuse box. Any loss downstream of battery voltage or higher than normal resistance to master. Is an issue that needs correcting.
Did you check the wires leads inside the boot, at bottom of ABS unit and at motor. If corrosion found, that is a bad sign (clue). Which I give the most negative weight of; -1 to -3 Depending on how badly corroded. New master I grade at 10, at a grade of 7. I start becoming concerned. Corrosion results in high resistance. Which is a killer, of the booster motor.
Here's another clue. With IG key ON, after booster motor stops running (fully pressured accumulator). Rapidly pump brake pedal, until alarm sounds. Count number of pumps. I find new brake master with 12.5 volts or better. Can take 10, 15 even more pumps, until ABS pressure alarm sounds. Weak old system, may be 7 or 5 pumps, 3 is very weak!
We can also have a voltage lose within ABS unit. We get same voltage to master, but not through ABS unit to wires leads at bottom in boot. With lower voltage, pump motor runs slower.
How many pump of pedal IG key OFF, until pedal pressure drops. Under 25 is good, 40 is bad. This is indicator of accumulator and or seal within master.
Also as noted. A leak, works booster system harder. Since booster systems must work harder to bring pressure back up, to overcome pressure loss of leak. These leaks are inside master, ABS unit or even accumulator.
Earlier this year. I had one with only 24K miles on it. Where brake alarms came on at different time and during different events. For one, almost always on first IG key in the morning. More frequent the colder the morning and longer duration. No leaks found externally at calipers, from lines or damage to flex lines (bubbles) or at master and wires in boots okay.
I found issue:
- Air in one rear brake line. Flushing & bleed. I saw improvement, reducing frequency of alarms at certain times when driving.
- With very little drop of battery voltage (by leaving on head lights). Alarms were even more frequent. I found oxidation and corrosion. At battery positive terminal under lug that bolts on the fusible link. Which was oxidation, from battery acid. Corrected and saw addition improvements.
- Fusible link and in wire housing blocks that connect to batteries fusible link, oxidation from acid. Corrected and saw addition improvements.
- Issues in fuse boxes with ABS relays. That being resistance at high end of limits. Corrected and saw addition improvements
As I work through the systems, correcting each issue, one at a time. I saw incremental improvements. But it still, I was getting ABS brake alarms. Just much less frequent. Ultimately, I was down to 3 possibilities. Fuse box (which had mud wasp nest internally), ABS computer or issue within brake master (not likely). My best guess was: Master Assembly fault.
Replaced the Brake booster w/Master ASSY (The whole brake Master ASSY). Problems solved.
Some find just replacing booster system or a component of (motor, pump, accumulator or wire). Solves the issue of morning IG Key ON ABS alarms. But all to often. We see failure on master side, within 2 years.
Most Dealership and I these days. Just replace the booster w/Brake Master ASSY (the whole deal). One and done.
