Parking brake tank pressure - need advice!

bevans6 · July 12, 2009, 12:36:34 PM

I'm reading the pressure at the emergency fill valve of my MCI MC-5C, and am reading zero with the bus up to full 120 PSI pressure. My schematic shows that valve directly connected to the parking brake tank. I can apply and remove the parking brake. When I drain the parking brake tank bethind the front wheels it has pressure. I removed the schraeder valve from the fill port, there is no pressure. Any ideas?

The reason I checked this was to check out the pressure protection valve. I wanted to be sure it was passing full pressure after the supply tank got up to 65 psi. My schematic flow shows air coming from the supply (dry) tank through the filter to the PPV, then to both the accessory tank, and to the parking brake tank though a check valve. The emergency fill valve is shown directly connected to the parking brake tank, and also to the dual brake valve, the push pull valve and the inversion valve through an 85 psi pressure regulator. That's a lot of important stuff to not be getting the right air pressure!

Thanks, Brian

Nusa · July 12, 2009, 01:12:39 PM

You mentioned a check valve. Is that positioned such that pressure readings aren't possible at that port? Or perhaps there's another check valve not on your diagram.

Another possibility is line blockage...has that valve accepted shop air previously? You should be able to tell if it's flowing.

bevans6 · July 12, 2009, 02:00:30 PM

I've just about decided that I probably leapt to a conclusion that the schraeder fill valve was also the one way check valve, but it may make sense that there is a second one way check valve in the line prior to the fill valve. I'll investigate that tomorrow. I will also investigate a way to put a gauge on the accessory tank. I also think I may have a faulty governor or something wrong with the compressor. The governor seems to either lag a lot after it's below cut-in pressure, or the compressor isn't holding air pressure in the wet tank and has to fill it before it can fill the dry tank. When I took pressure readings after an hour run, there was no air pressure at the ping tank, but pressure at the dry tank. I'm going to pop out and take readings now, it's an hour and a half since I last aired up the bus.

Brian

bevans6 · July 12, 2009, 02:34:13 PM

Well i have pressure in the wet tank, the parking brake tank and no pressure at the ping tank. Here is what I think is happening, please help me understand if I have this right! the compressor builds pressure up to cut-out. The governor cuts out and sends a signal to the air dryer to purge. The purge empties the air from the line between the ping tank and the air dryer, so there is no pressure between the compressor head and the air dryer. The one-way check valve in the air dryer maintains air pressure in the wet tank.

In other words, my system is working as it should be. I think. If I turned off the bus, stopping the compressor, while the compressor was building air (say 100 psi on the gauge) then I should see the pressure being maintained in the ping tank, since it would not have been purged off. I will try that tomorrow.

I am determined to figure out exactly how this system works!

Brian

gus · July 12, 2009, 06:28:18 PM

The ping tank (I call it a muffler) has no pressure when the comp is not running.

There is a check valve after it to keep air from bleeding back through the comp.

Even when it is running the pressure there jumps all over the scale.

Gary '79 5C · July 12, 2009, 06:29:48 PM

Bevans,

Do you have a schematic ? Not to sound like a smart a$$, but I should be able to dig on out from my maint manuals and email same.
I need to do so as I have a small leak in my tank in the driver side rear baggage compartment. Me thinks it is the remainder of the factory leveling valves which might be still pressurized.
This one system I have not conquered in knowledge, but need to do so.
I can air up, guage at 120, but I do not hear the governor "dump". I was inside the hanger with fast idle for 25-30 minutes.

Email me if you wish, if you need the schematic, garypasternak@aol.com

Take care,

bevans6 · July 13, 2009, 03:57:01 AM

Thank you, I have the factory maintenance and operators manual. I actually have a original, OEM as it were, operators manual for the 5C. That's where they call the discharge muffler a "ping tank". It does a real good job of accumulating moisture and oil out of the compressor, I drain it every time I drive the bus. My problem, if I have one, is that the schematic is incomplete. It doesn't show the air dryer, where pressure is read, the location of the switches, etc. That's fine, I'm getting to learn this.

The tank in the rear drivers side bay is the dry tank for the brakes only, it does not serve the suspension. Air is fed through a check valve, filter, pressure protection valve into the accessory tank then to the suspension. I would treat a known leak in the dry tank as fairly important.

Also, you can tell that the governor "dumped" if the air doesn't go past 120 PSI. it would continue to rise and trigger the safety valve at around 150 psi if the governor wasn't working. What you hear is the air dryer purging, which it does when signalled by the governor. If it isn't purging then maybe the signal line is bad or the air dryer is bad.

Tom Y · July 13, 2009, 04:16:28 AM

Gary, Brian is right about the tank in the rear bagage bay. For rear brake only, feeds air to a relay valve then to the rear brakes. When the pedal is applied air is sent to the relay valve opening it. Air from this tank then applies the rear brakes. Not a spot for a leak.. Tom Y

bevans6 · July 13, 2009, 04:41:19 AM

Air from the dry tank in the rear bay supplies all of the air for all of the brakes (on a 5C anyway), and supplies all the other tanks through various protection and check valves. It's directly connected to the R-8 relay valve which activates the rear brakes, but is also connected directly to the E-9 dual brake valve up front, worked by the pedal, from there it feeds the front service brakes through a quick release valve, and also feeds the signal line from to activate the R-8 relay valve. Unlike modern truck (or maybe coach) systems which have separate service tanks for front and rear brake systems, the DD3 system has all the brakes served from single tank for normal operation and a separate "parking brake" tank for emergency use that can feed air into the main system through the shuttle valve.

I still have no idea what the inversion valve does. Haven't got that far yet! Soon!

We should have an air system tutorial thread. I think there are a lot of people who could benefit from a deeper understanding of how things work and the air system is arguably the most important system on the bus!

Brian

Dreamscape · July 13, 2009, 04:56:04 AM

We should have an air system tutorial thread. I think there are a lot of people who could benefit from a deeper understanding of how things work and the air system is arguably the most important system on the bus!

I think you just started one!

Go here, you might get your questions answered. http://www.bendix.com/en-us/service/library/Pages/Home.aspx
http://www.bendix.com/en-us/service/library/Pages/Bulletins.aspx

Good Luck!

Paul

Tom Y · July 13, 2009, 05:32:28 AM

Brian, My bad about the rear tank. You are right. I had to go back to the book, should have went there first. My take on the inversion valve, for what its worth. Looks to me that it applies the park brake when the presure drops off. Tom Y

Gary '79 5C · July 14, 2009, 04:58:19 PM

Brian,

We should have an air system tutorial thread. I think there are a lot of people who could benefit from a deeper understanding of how things work and the air system is arguably the most important system on the bus!
You are very much correct, I need to correct this and like soon. I plan to get closer to this over the weekend.

I will keep in touch. I have had some confusion as to the schematic and the tank locations/ purpose.

I appreciate all the 35'ers assistance.

Gary

Gary '79 5C · July 15, 2009, 07:54:40 AM

I have three questions illustrated in the attached diagram.
I know these are basis questions, but I do not wish to make a poor assumption.

1. Is the Ping tank just off the air compressor, like within 2' ? A small 2" dia, 8" long tube, with threaded bottom with a blow down valve ? Wet tank as described in diagram.

2. Is my dry tank in the 5C is located in driverside rear baggage comp ??

3. R-8 Relay Valve is located on rear center bulkhead of rear baggage compartment.

Does the R-8 relay open/close with brake application? Does this also balance between the two rear brakes?
I need to look for the air leaks and I believe they start out from this Dry Tank. Sounds like dinosaurs snoring after shutdown.
Thanks,

Gary

bevans6 · July 15, 2009, 08:28:50 AM

1. Is the Ping tank just off the air compressor, like within 2' ? A small 2" dia, 8" long tube, with threaded bottom with a blow down valve ? Wet tank as described in diagram.

Yes, that is the ping tank, as they call it in the operators manual. Drain the ball valve on it's bottom daily, mine has a schrader valve to let me fill there from an external air supply.

2. Is my dry tank in the 5C is located in driverside rear baggage comp ??

Yes, that is the dry tank.

3. R-8 Relay Valve is located on rear center bulkhead of rear baggage compartment.

I believe so. the R-8 valve has two feeds from the dry tank, a signal line from the dual brake application valve (the "pedal valve" up front) and has an air linethat go, probably through a quick release valve, to the rear foundation brakes (the main part of the DD3's).

Does the R-8 relay open/close with brake application? Does this also balance between the two rear brakes?

The R-8 valve is what applies supply tank air pressure to the rear brakes. It gets the air directly from the dry tank. The signal line from the E7 foot valve tells it to send air to the brake cans. it sends the air to a quick release valve, which sends the air to the brake cans through identical length air hoses. The quick release valve is what splits the air to the cans and what vents the air to atmosphere when the brakes are released. Sending the air to the rear brake cans through the quick release valve and then through identical length hoses to the cans is what balances the brakes side to side. The front brakes are engaged through a quick release valve and identical length hoses too. The main thing this does is make sure the time lag to apply the brakes is the same side to side, The reason the R8 valve is directly connected to the dry tank is to ensure an adequate supply of air, and to reduce the time lag for the air pressure to find it's way from the front of the bus to the back of the bus. The signal line is quite small diameter compared to a main hose line, so the air signal travels quickly from the E7 foot valve to the R-8 valve.

I need to look for the air leaks and I believe they start out from this Dry Tank. Sounds like dinosaurs snoring after shutdown.

There is always pressure between the dry tank and the R8 valve. There is no air pressure after the R8 valve unless the brakes are applied. So check the R8 valve and those connections. Probably just replace the R8 valve if it hasn't been replaced lately, they are routine maintenance items. If you hear noise towards the front of the bus, look at the air dryer, sometimes the purge valve doesn't close.

I hope this helps you out. It is my understanding of the R8 valve, anyway. did this from memory, my 5C manual is at home.

Cheers, Brian

This site has some good stuff, this in particular is a guide to troubleshooting air brake systems. The R8 relay valve is number 14 in this diagram. Keep in mind the differences between a DD3 system and a spring brake system with primary/secondary tank system. http://www.todaystrucking.com/images/ArMerTruckTroubletp9974.pdf

Gary '79 5C · July 15, 2009, 09:10:51 AM

Many thanks Brian,

Just discussing this provides me with a clearer head to get the understanding I needed. My two holding tanks are just in front of the rear bulkhead, so it may time for me to remove the hot water tank, and lose 30 lbs to gain access & look between tanks. I have not looked for this valve before, but will tonight. I see also that the dry tank has a check in the feed line from the wet(ping) tank. I doubt that is leaking as I always have pressure at the blowdown.

Are there any tanks upfront under the steering & frt axle? and would that be the park brake tank?

Thanks again,

Gary