Servo rebuild question

dlyle

dlyle

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CAT Member
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I've got my servo all cleaned up but forgot which way the seals go on the piston. I've done several of these before but it's been a while so I want to make sure I get it right.

Does the taper on bot seals go the way as seen in the picture? I have a sneaking suspicion that at least the large one goes the other way. The seals that came off offer no clues as the taper is gone on both. I did pay attention when taking the seals off and the larger seal was flat on on side and not the other. If I put it back on in the same orientation the large seal will need to be flipped the other way.

Can anyone tell me for sure which way both seals go?

Thank you.
 
Think Pressure & Expansion

dlyle said:
I've got my servo all cleaned up but forgot which way the seals go on the piston. I've done several of these before but it's been a while so I want to make sure I get it right.

Does the taper on bot seals go the way as seen in the picture? I have a sneaking suspicion that at least the large one goes the other way. The seals that came off offer no clues as the taper is gone on both. I did pay attention when taking the seals off and the larger seal was flat on on side and not the other. If I put it back on in the same orientation the large seal will need to be flipped the other way.

Can anyone tell me for sure which way both seals go?

Thank you.
Click to expand...

FYI: The 5/8" seal in your photo is on backwards. The 7/16" seal is correct. The "lip" seals are designed so that the tapered "lip" faces the fluids and is forced outward during pressure cycles which increases the seal/friction against the bore. Ergo, the control piston seals face away from each other. Of particular attention is the power piston (the one attached to the long spring). This seal lip faces into the bore as does the one behind it between the fluids and the vacuum can. The power piston seal may/can roll during installation and create a non-boost problem. Test install it several times with lube and check it for direction and smooth operation before continuing assembly.
 
And another thing.......

When re assembling the control piston in the bore use brake fluid as a lube rather than rubber grease. Due to variations in seal design/suppliers some seals have too much friction when assembled and will not return properly via the spring shown in your pic. Rubber grease will mask this problem during assembly but once the car is on the road and the grease has been dispersed you may find the servo assistance is far from subtle due to the control piston dragging in it's bore.

Also, while in years past the foam filler strip for the vac piston has been too thick causing the brakes to drag, I recently inherited a rebuilt servo where the vac piston had too little friction in the bore of the vac housing, also causing the assistance to be unacceptably abrupt. Fitment of the foam seal from the Powertrack kit (unused by the previous "brake specialist") solved the problem but not before many hours of road testing and head scratching.

The end result was a Tiger with all the correct bits under the bonnet that drives with all the idiosyncrasies of the original.....and a happy owner. ;)

Lance.
 
Some are cutting the "backer rod" in half, but I think the better option is to go get a small rod from the local hardware supplier.

http://www.amazon.com/Closed-Cell-Backer-Rod-Roll/dp/B001OBTZ1E

anyone want to share a roll!

:)

moonstone SIV said:
And another thing.......

When re assembling the control piston in the bore use brake fluid as a lube rather than rubber grease. Due to variations in seal design/suppliers some seals have too much friction when assembled and will not return properly via the spring shown in your pic. Rubber grease will mask this problem during assembly but once the car is on the road and the grease has been dispersed you may find the servo assistance is far from subtle due to the control piston dragging in it's bore.

Also, while in years past the foam filler strip for the vac piston has been too thick causing the brakes to drag, I recently inherited a rebuilt servo where the vac piston had too little friction in the bore of the vac housing, also causing the assistance to be unacceptably abrupt. Fitment of the foam seal from the Powertrack kit (unused by the previous "brake specialist") solved the problem but not before many hours of road testing and head scratching.

The end result was a Tiger with all the correct bits under the bonnet that drives with all the idiosyncrasies of the original.....and a happy owner. ;)

Lance.
Click to expand...
 
0neoffive said:
FYI: The 5/8" seal in your photo is on backwards. The 7/16" seal is correct. The "lip" seals are designed so that the tapered "lip" faces the fluids and is forced outward during pressure cycles which increases the seal/friction against the bore. Ergo, the control piston seals face away from each other. Of particular attention is the power piston (the one attached to the long spring). This seal lip faces into the bore as does the one behind it between the fluids and the vacuum can. The power piston seal may/can roll during installation and create a non-boost problem. Test install it several times with lube and check it for direction and smooth operation before continuing assembly.
Click to expand...

Thanks for the info. I've got it correct now.
 
moonstone SIV said:
And another thing.......

When re assembling the control piston in the bore use brake fluid as a lube rather than rubber grease.

Also, while in years past the foam filler strip for the vac piston has been too thick causing the brakes to drag, I recently inherited a rebuilt servo where the vac piston had too little friction in the bore of the vac housing, also causing the assistance to be unacceptably abrupt. Fitment of the foam seal from the Powertrack kit (unused by the previous "brake specialist") solved the problem but not before many hours of road testing and head scratching.


Lance.
Click to expand...

I always use Mckay assembly lube when putting hydraulics back together. I also never use the thick foam seal. Rick at Sunbeam used to take out the thick seal and substitute a smaller diameter foam seal that worked well. I've also left them out and used the original with great results.
 
Not sure where Rick sources his rebuild kits, the above info is based on the items supplied by Powertrack in the UK. They often have listings on ebay etc.

The foam packing in the 7" kit certainly appears too large, my post above relates to the assembly of a 5.5" unit and the foam supplied was no more than 1/4" in diameter. The vac piston when removed had a strip of graphite impregnated rope as a packer which had obviously collapsed over time allowing the piston to move too freely in the bore of the housing.

Can anyone enlighten us as to what was used as an OE packing strip by Girling 50 years ago?
 
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