Low line pressure in reverse - causes

[hjelmsd]

My 727 has had a slipping reverse gear for some time. I finally started diagnostics. I get only 180-190 psi when checking the reverse servo line pressure. Service manual indicated 240-270 psi. So I have low pressure, which I assume is consistent with my reverse problem. I also checked the low/reverse port pressure while in Low 1 gear. Measured 55 psi, just about right per the manual. I did these checks while transmission was relatively cold, wheel spinning around 1600 - 2000 rpm.

Based on my research and manual, I am thinking there are 2 possible causes to diagnose: low pump pressure (failing pump) or valve body issue. I found another valve body that I will swap in and test to rule out valve body. If I still have low reverse line pressure with the other VB, I will concluded the issue is the pump.

I haven't bothered to check line pressure on the other ports yet, but can if it's likely to be helpful. 2-3 shift and operation in 3rd gear (1:1) has been fine I think. All forward operation has been fine, but I just cruise around.

Any other likely root causes to further diagnose?

Data: Engine is 383, 2400 stall converter, transmission is 1968 727, valve body is factory style for this year. The issue first appeared after my rooster comb assembly loosened and I jammed the rooster comb against the valve body casting - jammed into park. I fixed the comb assembly, but think maybe the comb arm had the manual valve loaded/jammed up and maybe damaged the VB (it wasn't obvious)

I made sure I am in the reverse detent location, ruled out shifter linkage variation.

Thanks for your help,
Scott

 

[Lefty71]

100% sure level is correct? Idle warm in neutral to be sure. Any foaming or bubbles in fluid could also indicate low fluid or a leak (internal) possibly with the valve body.

 

[MWbirdLOVER70]

My 727 has had a slipping reverse gear for some time. I finally started diagnostics. I get only 180-190 psi when checking the reverse servo line pressure. Service manual indicated 240-270 psi. So I have low pressure, which I assume is consistent with my reverse problem. I also checked the low/reverse port pressure while in Low 1 gear. Measured 55 psi, just about right per the manual. I did these checks while transmission was relatively cold, wheel spinning around 1600 - 2000 rpm.

Based on my research and manual, I am thinking there are 2 possible causes to diagnose: low pump pressure (failing pump) or valve body issue. I found another valve body that I will swap in and test to rule out valve body. If I still have low reverse line pressure with the other VB, I will concluded the issue is the pump.

I haven't bothered to check line pressure on the other ports yet, but can if it's likely to be helpful. 2-3 shift and operation in 3rd gear (1:1) has been fine I think. All forward operation has been fine, but I just cruise around.

Any other likely root causes to further diagnose?

Data: Engine is 383, 2400 stall converter, transmission is 1968 727, valve body is factory style for this year. The issue first appeared after my rooster comb assembly loosened and I jammed the rooster comb against the valve body casting - jammed into park. I fixed the comb assembly, but think maybe the comb arm had the manual valve loaded/jammed up and maybe damaged the VB (it wasn't obvious)

I made sure I am in the reverse detent location, ruled out shifter linkage variation.

Thanks for your help,
Scott

Is there a band adjustment? I can’t remember

 

[hjelmsd]

Thanks. Good reminder that low level causes low pressure. I am certain that it's full.

I am not aware of any reason that the pressure would change with any band adjustment.

 

[oldkimmer]

The seal in the front drum could have a minor leak. U won’t know till u check the pressure. Also the seal on the reverse piston could have a leak. Or u may have bent the manual valve. Or your slopped out linkage on the valve body is causing problems. Kim

 

[GTX JOHN]

I seem to recall that it requires more pressure to engage Reverse
than the forward gears. Usually has been Pump problems that shows
up first in reverse gear for me.

 

[dvw]

I've never seen a pump worn enough to cause low pressure. The suspect low pressure port also is used for 3rd gear. Does it slip in 3rd? How does the pressure look in that port in 3rd gear? If it's good I' say it has a issue with the the valve body. If its low then there could be a reaction shaft ring or front clutch piston seal issue. Sonnax has a fix for the valve body. Probably easier just to swap one in if the diagnosis leads you there.
Doug

 

[hjelmsd]

My diagnosis continues. My rear band is loose. Loose to the extent that could not adjust it to spec. I removed the servo arm/pivot so that I can inspect the band while installed and transmission is in the car.

I am able to use large jaw pliers to pinch the band closed so each end touched. When I do this it does grip the drum, but the ends are in contact. Can someone please confirm that I should not be able to do this? And therefore, I need a new reverse band....to worn. That's what I think my issue is.

Thanks in advance.

 

[dvw]

The band has to be worn out if you can clamp it and itge ends touch. But I can't see this causing the low pressure issue at the reverse servo port. I think it's time form disasembly and inspection.
Doug

 

[oldkimmer]

Any updates? Kim