My better half was driving our 1988 Class B today. It's got the Dodge Ram Van 1-ton chassis, with the 5.9L V8 engine. It has about 65,000 miles on it. She called me and told me that the "BRAKE" warning light had illuminated, and that the brake pedal had gone to the floor with little to no braking power. Luckily she was in a non-traffic situation at the time.

I called my older son (who used to be a Dodge mechanic) after my better half made it home. He told me to check the brake fluid level, and it was fine. He told me to look at each wheel to check for brake fluid leaks, and there were none. He told me to look and listen for any vacuum leaks around the engine, and I found none. The engine had been running smooth as silk. He then told me to turn the ignition key back on, and "pump" the brakes 5 times to bleed off the vacuum, then apply slow, steady pressure to the brake pedal to see what happened. After bleeding off the vacuum, the brake pedal slowly but surely sank down to the floorboard, and the "BRAKE" warning light illuminated. Starting the engine, and pumping the brake pedal, firmed up the brake pedal again, and the "BRAKE" warning light went off. My son said that it sounded like the classic symptoms of a master cylinder going bad due to an internal leak.

Before I spend any money I wanted to check this forum to see if there were any other theories out there. Does a bad master cylinder sound right?


UPDATE: It's fixed. A new master cylinder was installed today and the brake system was flushed. The brakes are now back to normal. Thank you all for your helpful advice and suggestions.

 

Sounds like a possibly failure, does this vehicle have ABS? if not pinch off all 4 wheels at rubber lines to see if height increases, if not the problem does lie in the Master. If pedal becomes hard, undo one line at a time and see which wheel is the problem. If you have ABS, I believe you would have the rwal, their are methods for testing the accumulator inside that unit.

 

Just in case you still have problem here is a bit extra information

If you are sure that there is no brake fluid leak anywhere in system......

MOST brake systems have..the power booster and the master cylinder.
The power booster usually is run by vacum and its purpose is to minimize
the travel needed on brake pedal to complete stopping.
Usually you can test BOOSTER by pumping brake pedal 3 - 4 times with
ignition key in OFF position....pedal should become very stiff no movement.
If pedal begins to drop, it USUALLY indicates a vacum leak.
If pedale is stiff, when you turn ignition on, the pedal will drop slightly
toword floor ..that is normal

The master cylinder is mounted to power booster. So the two work
together but are individual units. There should be a valve on outside
of BOOSTER unit and it should have a hose heading toward engine.
Check hose and valve, also connection at engine.

If you do have ABS brake system too, USUALLY , the normal system
and ABS will only put on their respective warning lights.
The normal system handles braking in general, the ABS provides more
controled stopping in hard braking situations
If normal system is failing it usually will show brake failure light.
If ABS light is on it PROBABLY means ABS has failure BUT normal
ssytem should be ok.

The brake sytem is something you do not want to take
chances with.

 

If you have an internal leak or faulty accumulator spring in the HCU. The ABS will not set a code with the older style diagnostics.

 

UPDATE: It's fixed. A new master cylinder was installed today and the brake system was flushed. The brakes are now back to normal. Thank you all for your helpful advice and suggestions. (This old van did not have ABS on it...)