Rear brakes lock-up

MOTORHEAD · #1 (**permalink**) 12-01-2015, 06:57 AM

It would appear my rear brake master cyl is not releasing pressure when you release the pedal. This is in the original style pedal box set-up. I'm pulling things apart to gain access (not easy), but thought I'd post here to see if anyone with a Kirkham or original had a similar experience and might have some helpful tips.

Ted

cycleguy55 · #2 (**permalink**) 12-01-2015, 09:12 AM

One possibility is a problem with the residual pressure valve. They're installed whenever the master cylinders are lower than the brake caliper and, for disk brakes, supposed to retain 2 PSI in the lines. If that valve is sticking or defective it won't allow the pressure in the rear brake lines to drop after application of the brakes.

It will look like this picture (or very similar) and be installed inline on the line between the MC and the rear brakes.

I would think they're pretty simple and reliable, but perhaps some debris was in the system and lodged in the valve. I don't know if they're repairable, but they're relatively inexpensive and easy to replace, though you will need to bleed the rear brakes when you're done.

rodneym · #3 (**permalink**) 12-01-2015, 09:15 AM

No problem with brakes here but I did replace my clutch master. I remember it being a pain. I took a wrench, heated and bent it so I could reach around the cylinder to the higher bolt. Its the the typical Cobra job - simple but hard to get to.

Morris · #4 (**permalink**) 12-01-2015, 10:02 AM

If you have a Residual valve in the rear brake circuit.......remove it ...it is not needed and can cause issues all of the time.....

Tommy · #5 (**permalink**) 12-01-2015, 10:54 AM

I know nothing about Kirkhams, but if it has a balance bar setup, I'd check that.

DanEC · #6 (**permalink**) 12-01-2015, 11:37 AM

Quote:

Originally Posted by Tommy

I know nothing about Kirkhams, but if it has a balance bar setup, I'd check that.

And make sure your brake pedal is returning fully upon release and not hung up on carpet or something.

Can't see why a residual pressure valve would be necessary.

cycleguy55 · #7 (**permalink**) 12-01-2015, 11:53 AM

Okay, never mind.

I just looked at one of the Kirkham documents (http://www.kirkhammotorsports.com/book_aoe/aoe_16.pdf) and believe there is NO residual pressure valve installed.

Today I learned something, so it's a good day.

MOTORHEAD · #8 (**permalink**) 12-01-2015, 12:59 PM

Just got back home. Thanks for the replies. No residual valve. Need to remove carpet,& sound/heat mat to get top pedal box cover off. Think it's the bottom cyl. May need to remove inner wheel well panel to access lines.

Ted

rodneym · #9 (**permalink**) 12-01-2015, 04:16 PM

Ted, can't you just open the panel at the bottom?
I don't remember off hand it being that crazy down there.

MOTORHEAD · #10 (**permalink**) 12-01-2015, 06:36 PM

Rodney: The bottom plate allows some access, but not enough. Need to hold top bolt head which is accessable thru the top plate, remove vent to access sidepipe bolts, remove side pipe to see what I was doing fwd of the box, disconnect lines. That all allowed me to remove the MC. Found area outboard of the piston was contaminated with some rusty coating and jamming the piston from returning to the "off" position. Will try to post photos later

Ted

MOTORHEAD · #11 (**permalink**) 12-02-2015, 07:06 AM

Some pics, I hope!

lovehamr · #12 (**permalink**) 12-02-2015, 07:22 AM

Wow, and I thought BDRs were a hassle!

cycleguy55 · #13 (**permalink**) 12-02-2015, 08:01 AM

Those photos clearly show low-mounted master cylinders, different than I saw yesterday in the Kirkham document I looked at (http://www.kirkhammotorsports.com/book_aoe/aoe_16.pdf) - perhaps that document relates to later production.

Regardless, given the low mount, there s/b a residual pressure valve somewhere in the rear brake line, unless there's one built in to the MC. Even if it's not the source of the current problem, it's worth knowing where it is.

MOTORHEAD · #14 (**permalink**) 12-02-2015, 08:29 AM

If I remember (which I don't do very well anymore)correctly, working thru the side vent hole was the trick on theBDR's.

Brian: believe me, I have re-built this car down to bare chassis/body (well, partial body), and there is no residual pressure valve on it anywhere. Besides, the sticking MC piston was the culprit. This is a very early Kirkham, sn 0028, built in 1998.

MOTORHEAD · #15 (**permalink**) 12-02-2015, 08:35 AM

Brian: the car in the book you referred to is the Larry Elison "billet" Kirkham, built for 1mil
a few years ago. A totally different animal !

cycleguy55 · #16 (**permalink**) 12-02-2015, 08:49 AM

Quote:

Originally Posted by MOTORHEAD

Brian: the car in the book you referred to is the Larry Elison "billet" Kirkham, built for 1mil
a few years ago. A totally different animal !

"A totally different animal!"

Agreed - both the car and Larry.