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Tom Norman · 01-24-2006, 02:36 AM

My better half was driving our 1988 Class B today. It's got the Dodge 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?

Tom Norman · 01-24-2006, 02:36 AM

My better half was driving our 1988 Class B today. It's got the Dodge 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?

Joe-K · 01-24-2006, 03:00 AM

The diagnosic troubleshooting steps your son had you step through sounded thorough. If the pedal is going to the floor, the fluid that the master cylinder piston displaces is going somewhere. Bleeding off the vacuum so that it's easier to monitor the pedal with constant foot pressure allows you to see if the pedal is moving downward.

Looking for fluid leaks at the wheels including checking the wheel cylinders for leaks would be the first step. I'd also grab a mechanics creeper and roll along under the rig following the brake lines from the master cylinder to each wheel. With the salt put on roads today, it's very possible to have the steel brake lines rust and develop pinholes. By examining each line, you'll spot a wet spot on or underneath the brake line if you have a leak. Once you trace out all the steel brake lines, carefully check the rubber lines that attach between the wheel brake cylinders or calipers and the steel lines. Often times those lines harden and crack with age.

If the pedal drops and there is no fluid leak, most likely the master cylinder is bad.

NeilV · 01-24-2006, 05:42 AM

Tom,

I've personally rebult enough master cylinders to say your son is most likely correct. On an 18 Year old vehicle if you have not already replaced the wheel cylinders, calipers and rubber lines yet, it would be a great idea to do it all now. No matter how well they flush the system if your hoses and seals are breaking down on any of the other components the particles could still migrate back to the master cylinder and cause problems in the future. If you have driven in road salt then the steel lines should be inspected and any that are showing signs of weakness should be replaced also.

Play it safe.

Just went through this on my 85 Plymouth Voyager that my youngest daughter wants to drive and started at the master cylinder and if it was rubber, rusted (beyound a light surface rust) or had moving parts I replaced it. I am still able to do the work myself so there was quite a savings on labor and all the parts I used carry a lifetime warrenty.

Regards,

Neil

Mac and Chris · 01-24-2006, 08:36 AM

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">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. </div></BLOCKQUOTE>

Yep, it's the master cylinder. Sure as can be. No leak, so there's no place for that pressure "escape" but internally.

Mac

Tom Norman · 01-25-2006, 01:41 PM

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.