IH PARTS AMERICA
Click Here!

Go Back   IH PARTS AMERICA > Tech Forums > Brake Tech
Register FAQ Members List Calendar Search Today's Posts Mark Forums Read Photo Gallery IH Store Home

Reply
 
Thread Tools Display Modes
Old 08-09-2017, 04:11 AM   #1
meisterj
Member
 
Join Date: Jan 2017
Member Number: 8515
Posts: 30
Default 4W disc master cylinder plumbing

After brake, suspension, and bearing work at all four corners I am now ready to install the new MC, combo valve, and booster for the 4 wheel disc brake system (did a rear conversion) on a '74 scout II. I have the correct components and inquired about the procedure for proportioning/combo valve installation as this is my first brake work other than changing pads. I was told to be sure the front MC reservoir is plumbed to the rear brakes, and back reservoir to front brakes. What is the reasoning for this? I thought the combo valve took care of all the pressure distribution issues, and as I recall the two MC reservoirs are the same size (although I forgot to confirm this prior to leaving for work this am). Is there a difference in function between the two reservoirs?
meisterj is offline   Reply With Quote
Old 08-09-2017, 08:25 AM   #2
FDChappie
Senior Member
 
FDChappie's Avatar
 
Join Date: Aug 2011
Member Number: 3742
Location: Northern New California
Posts: 1,516
Default Re: 4W disc master cylinder plumbing

I'm sure someone here will whack me in the knuckles with an iron rod if this is wrong :-). I think the reason for plumbing this way is that as you step on the pedal, the back chamber is pressurized first. Since most of the braking force, 70%, is achieved by the front brakes, it is a lot less likely that the front brakes will lock up before the rear.

Even plumbed this way on my '74 Scout with OEM disk brakes, the rear drums will lock first when the brakes are applied quickly.
__________________
1974 Scout II 345 T-19
1971 Travelette 392 T-36
1972 Loadstar 1700 dump truck 345 T-35
FDChappie is offline   Reply With Quote
Old 08-11-2017, 03:56 PM   #3
Greg R
Senior Member
 
Join Date: Jul 2008
Member Number: 527
Location: Lebanon, OR
Posts: 757
Default Re: 4W disc master cylinder plumbing

Quote:
What is the reasoning
Chappie is correct. The rear piston of the MC is the primary and the 1st to pressurize. Once it's past its compensating port, the pressure also acts on the front secondary piston to move it to pressurize the rear brakes. There is a rod that limits travel or can connect the pistons during a system fail, but it's pressure that does all the work in a functional maintained system
Greg R is offline   Reply With Quote
Old 08-13-2017, 03:00 AM   #4
meisterj
Member
 
Join Date: Jan 2017
Member Number: 8515
Posts: 30
Default Re: 4W disc master cylinder plumbing

Ok, thanks guys!
meisterj is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off



All times are GMT -8. The time now is 03:59 AM.