| Author | Message | ||
| Jim-Bob (12.46.52.74) Rating: |
I have a '76 4905. I have a 1975 manual for 4905. The pictures & air schematics in the manual are not the same as the stuff in the bus. The problem is that my bus has a relay valve on the rear air tank which operates the parking & lock system on the DD3 cans. "Da Book" shows an inversion valve in the DD# brake system with no relay valve. The relay valve appears standard due to the mounting & tubing arrangement. Did GM change their system sometime between '75 & '76? Does anybody's manual show the DD3 system with a 2 relay valves? BTW, I looked under an Eagle & the use 2 relay valves so it's not that odd. Dazed & Confused | ||
| Rayshound (12.34.243.185)
Rating: N/A |
Jim; I have an mci-8 with the dd3 system with two relay valves in the rear. I have two r-6 valves. Appears to be factory mount. the second relay valve is for the tag axle. it does not show up in the mci p&id. Ray | ||
| Jim-Bob (12.46.52.74) Rating: N/A |
Well the GM book discusses a second valve for tag brakes. But this bus doesn't & never had a tag. So your MCI-8 has an inversion valve to control the lock ports & park ports of the rear drive axle brake cans? | ||
| John the newbie (199.232.240.75) Rating: N/A |
FWIW: "The next version, the PD-4905 generated only 331 sales. The P8M-4905A introduced an optional fully-retractable third axle; 2,027 were sold in seven years of production. The GMC H8H649 was a continuation of the GMC P8M4905A and comparison of the manuals has shown no difference between the coaches." From: http://www.coachinfo.com/AllAboutBuses/GMC_Info.html | ||
| J.L.Vickers (209.34.27.250) Rating: N/A |
Da Book" shows an inversion valve in the DD# brake system with no relay valve. The relay valve appears standard due to the mounting & tubing arrangement. What number and year is you GMC 4905 maintenance manual? My P8M4905A maintenance manual is X-7564 dated 1-75 It shows the relay and quick release valve it the Schematic Diagram of DD3 brake actuator system Section 4 page #31. And yes the relay valve is mounted on the bulkhead under the rear air tank. Section 4 page 18 figure # 3. J.L.Vickers | ||
| Jim-Bob (12.46.52.74) Rating: N/A |
Hi J.L., My manual is 1975. Can't tell # & pub date 'till tonight as it is home. I think the date is 1/75 but will confirm. My 4905 has the relay valve UNDER the tank which controls the service brake ports. But it has another relay valve mounted on a bracket on the air tank which controls the parking/emergnecy & the lock ports on the DD3s. There are also about 5 other small valves & check valves in this location which do not appear in the air brake component photographs. All of the tubing & hoses connecting this stuff look like original GM. There was a proportioning valve installed on the service brake relay valve which the book showed as only being used in the tag brake system and it was very obvious from the twisted plumbing that it did not belong there. I'm hoping somebody's manual will have a more detailed section covering this part of the system. Sure like the 4104 book better. The systems are simpler AND better documented. | ||
| J.L.Vickers (209.34.27.250) Rating: N/A |
Yes the old PD-4104's were a lot simpler and easier to maintain as the systems were not so complicated as the later model GMC Coaches. Now just think if we had a New MCI or Preavost we would look back and oh I wish I had my old GMC Buffalo when some system went wrong. J.L.Vickers | ||
| TWO DOGS (4.227.115.227) Rating: N/A |
just had a friend replace an ignition switch on a '98 buick....150.00...good ol' days long gone... | ||
| Buswarrior (Buswarrior) (64.229.211.228)
Rating: N/A |
Hello Jim-Bob. The feds changed the rules during 1975 as to the plumbing of the air brake systems and their performance. A 1975 manual may not have the required dual circuit schematics in it, and be of no use to your 1976 air system. Basically, prior to 1975, the service brakes were allowed to be single circuit, after 1975, they have to be dual circuit, typically one isolated tank for the front axle, one for the back. The purpose was to build in a bit of redundancy and safety. One must be careful with buses built during 1975, as they might be one way, or the other, or a hybrid built under an limited exemption to parts of the new rules. A bit of trivia, for the MC8, built 1973 to 1979, there are 8 different air schematics in the last manuals depending on jurisdiction, (plain, NJ or MA safety specs) pre and post dual circuit, anti-skid control or not. Wow, what fun! My MC8 DD3 schematics are away with the bus, so, I'm winging it: the inversion valve may be part of a control circuit, the actual volume of air may be directed to the chambers from the tank using a relay valve, not via the control valving. Let us know how you make out! happy coaching! buswarrior | ||
| Jim-Bob (205.188.116.135) Rating: N/A |
Hi Buswarrior, Well your answer certainly helps explain this. The bus has the dual front & rear tanks/system. It may be that the inversion valve is there too. I guess I need to start by finding somebody who has a 4905A manual from 1976 or newer that will have the correct configuration. How about it guys, anybody got a '76 4905A manual showing two relay valves at the rear for the DD3 brake system? | ||
| J.L.Vickers (209.34.27.250) Rating: N/A |
Jim-Bob I found my GMC Coach Maintenance Manual dated 3/78 Manual number C-7713 Section 4A-2 air system Scematic shows 2 relay valves One is a brake relay valve other is the parking brake relay valve. Both located in the rear bulkhead area. So what you have is the later style brake system different that the 1975 edition maintenance manual shows. Does your bus have two needles on the air pressure gage? One is green and the other is yellow? J.L.Vickers |
