Author Topic: Hydrovac fix/modification  (Read 206 times)

8683jb

  • Hero Member
  • *****
  • Posts: 593
Hydrovac fix/modification
« on: February 04, 2020, 08:39:48 PM »
Some time back, I rebuilt my sitting-for-decades hydrovac using an NOS kit I got from Dave Yamulla. It worked great for a short while but then about 3 or 4 days later the master cylinder was empty, and the pedal was on the floor. No external leaks anywhere; the fluid had transferred into the vacuum cannister.



Hmm… I tore it all down and disassembled the slave cylinder again to make sure everybody was installed correctly and tried it again. Same thing happened – good for a few days, then fails. Evidently this is not uncommon with these particular units and there is a fix for the slave cylinder that involves machining something and installing an improved seal somewhere. None of the rebuilders would give any details about the repair though; they would only include it in a complete overhaul for 650 to 700 bucks - plus ridiculous shipping costs there & back. Well I didn’t want the whole unit overhauled; I only wanted to send the slave cylinder to get the seal modification done – the rest had just been overhauled and was working fine.
Then I tried the other option, which was to buy a later hydrovac and build mounts for it and be done. No one has any. Apparently, they’re core-reliant and there are no cores coming in at this time. I had paid for one back in November and after two months with still nothing in sight, I finally cancelled the order and thought about finding out why mine leaks.

I called an outfit called Karps Power Brake in Upland, CA. (http://www.karpspb.com/) and spoke to a fellow named Jeff. What a great help!! He spent several emails and phone calls with me trying to get to the bottom of it. After sending him some pictures and some scans of the Tech Manual, he suspected that the trouble was with the atmospheric valve piston. After studying the schematic of the system, it made perfect sense. When the engine is running, vacuum is always present on the control valve side of the piston. Brake fluid is always present on the slave cylinder side of it. There is no seal on the piston, and with the wear on the piston and its bore over the years, there becomes enough clearance to allow brake fluid to pass into the control valve body and then into the vacuum canister.



I thought I’d try simply installing two o-rings on the piston to see if that would cure it. I cut two grooves in the piston to accommodate two ½” O.D. x 3/8” I.D. EPDM o-rings.





I reassembled the slave cylinder and after disassembling and cleaning the vacuum cannister and all the vacuum components, mounted it back on the half-track and bled the system. It’s been two weeks since the unit was reinstalled and there’s still a firm pedal (I’m delighted there’s ANY pedal) and the half-track stops perfectly.

I hope I haven’t spoken too soon, but this seems to have fixed mine and it all cost less than $16.00 (I only needed two o-rings but had to buy a bag of 100 from McMaster-Carr).

Many, many thanks to Jeff at Karp’s Power Brake for all the time & trouble! It’s so refreshing to find a company who’s willing to help a guy and not just try to generate business.

Jon



« Last Edit: February 10, 2020, 12:53:53 PM by 8683jb »
'42 Autocar M2A1

MVPA member

Tapper02

  • Sr. Member
  • ****
  • Posts: 443
Re: Hydrovac fix/modification
« Reply #1 on: February 04, 2020, 09:08:01 PM »
....but this seems to have fixed mine and it all cost less than $16.00 (I only needed two o-rings but had to buy a bag of 100 from McMaster-Carr).

Jon

And the cost of a lathe!  ;D

j/k...that is fantastic Jon.  Glad it worked out and I'm sure this will help someone down the road.  Never ceases to amaze me how you come up with some of your "fixes". 

-Tom
1944 AUTOCAR M15A1 (M16A1) HALF-TRACK:
  (Being restored to an M3A1 configuration)
  SERIAL NUMBER M-15 A 1, 1356
  MODEL M-16 A 1, ORD. SERIAL NUMBER 1106
1944 Schelm Bros. M10 Ammunition Trailer
  SERIAL NUMBER 16959
MVPA # 30507

8683jb

  • Hero Member
  • *****
  • Posts: 593
Re: Hydrovac fix/modification
« Reply #2 on: February 05, 2020, 08:16:47 AM »
Thanks Tom. Hopefully it'll stay fixed!
'42 Autocar M2A1

MVPA member

yd328

  • Hero Member
  • *****
  • Posts: 647
Re: Hydrovac fix/modification
« Reply #3 on: February 05, 2020, 08:52:35 AM »
Nice fix Jon,
I would like to put my original hydro vac back on. The updated one works fine but I like the original. I'll have to add it to the to do list. I have a lathe, I just have to figure out how to use it. ;D

Gary

col.halftrack

  • Sr. Member
  • ****
  • Posts: 357
Re: Hydrovac fix/modification
« Reply #4 on: February 05, 2020, 09:18:53 AM »
Great Post! Thanks Jon.
1941 White M2
1942 White M4A1 was M4
1942 Autocar M2A1
1942 Autocar M3-75
1943 White M13 rebuilt as M16
1943 Diamond T M3A1

big44maghunter

  • Full Member
  • ***
  • Posts: 152
Re: Hydrovac fix/modification
« Reply #5 on: February 05, 2020, 06:17:48 PM »
Good fix Jon!  Hope it's permanent for you.  Then you'll have to put a price on a mail order job for the rest of us!

8683jb

  • Hero Member
  • *****
  • Posts: 593
Re: Hydrovac fix/modification
« Reply #6 on: February 05, 2020, 07:23:16 PM »
Thank you guys. So far, so good on the fix but time will tell. I have scads of extra o-rings if anyone needs them and can't find them locally. I can get neoprene and Buna-N o-rings all day long, but no one near me stocks EPDM ones.

'42 Autocar M2A1

MVPA member