noise from vicinity of master brake cylinder

[Jim McGarvie]

Our Knight has developed a high-pitch squeal from the vicinity of the master brake cylinder when depressing the brake pedal with the engine running. We have an air-over-hydraulic system, and the mechanic at the local Big O thinks it is an air leak in the booster. I think he is right. Any other ideas? Where should I look for a replacement, or is it something that can be fixed (new seal or something)?

Thanks.

Jim

[Jim McGarvie]

Update on my post: I am embarrassed to admit after owning the coach since it came off the assembly line that we do NOT have air-over-hydraulic brakes. I don't know why I always thought we did. We have straight hydraulic brakes boosted by pressure from the power steering pump, and backed up by an electric pump mounted on the master cylinder.

So obviously the squeal we hear is not an air leak. With this new information, anyone have any ideas?

Thanks!

Jim

[Frank McElroy]

Many years ago I had a 1989 40' Winnebago tag axle gasser with an engine operated hydraulic boost brake system.  The electric pump was the backup system so you would have power brakes if the engine stalled.  With the engine off, if you press hard on the brakes with the ignition on, you should hear the electric boost motor.  If you are hearing the same electric boost motor running while the engine is on, that means that your engine brake boost pump is not working properly.  Possibly a loose belt or low fluid.  At least that's how the system operated on my old coach.  The system was OK but it was like driving a car without power brakes.  This was a major reason why my next coach was a diesel pusher with air brakes.

[Jim McGarvie]

4 minutes ago, Frank McElroy said:

Many years ago I had a 1989 40' Winnebago tag axle gasser with an engine operated hydraulic boost brake system.  The electric pump was the backup system so you would have power brakes if the engine stalled.  With the engine off, if you press hard on the brakes with the ignition on, you should hear the electric boost motor.  If you are hearing the same electric boost motor running while the engine is on, that means that your engine brake boost pump is not working properly.  Possibly a loose belt or low fluid.  At least that's how the system operated on my old coach.  The system was OK but it was like driving a car without power brakes.  This was a major reason why my next coach was a diesel pusher with air brakes.

Thanks Frank. I do have the electric pump back-up system, and it is easy to hear it operate when depressing the brake pedal with the engine off and ignition on. But our back-up pump is not running when the engine is running. That is not what I hear. What I hear is a high-pitch squeal coming from the vicinity of the master brake cylinder. It is probably some kind of an internal leak of the hydraulic boost, which is pressurized by the power steering pump.

[Frank McElroy]

OK, you have a hydro-boost system.  Yes there is valving inside the boost cylinder.  Unfortunately I gave away all my troubleshooting and service manuals to the new owner when I sold my old coach.  I did a quick search online and I see a lot of technical information on your hydro-max booster including replacement parts and rebuild kits if your inclined to tackle the project yourself.  Otherwise a heavy duty truck shop that does hydroboost brakes should be able to fix the problem.  I'm assuming that you already checked the hydraulic fluid level and other that this new noise, the brakes are operating normally with no increased pedal effort to stop.

[Jim McGarvie]

On 10/6/2020 at 8:19 PM, Frank McElroy said:

OK, you have a hydro-boost system.  Yes there is valving inside the boost cylinder.  Unfortunately I gave away all my troubleshooting and service manuals to the new owner when I sold my old coach.  I did a quick search online and I see a lot of technical information on your hydro-max booster including replacement parts and rebuild kits if your inclined to tackle the project yourself.  Otherwise a heavy duty truck shop that does hydroboost brakes should be able to fix the problem.  I'm assuming that you already checked the hydraulic fluid level and other that this new noise, the brakes are operating normally with no increased pedal effort to stop.

Thanks for the info, Frank. Yes, I did check the fluid level, and the brakes are operating normally.