Grabbing brake

[Momo]

Well, I'm a little stumped. I need the collective experience of the forum to help me figure this one out! Bear with my long post here...

I have a 1960 SII 109 SW and the front right brake is grabbing... in fact I can lock it up and skid pretty easily. At first it only happened with the first few applications of the brake pedal... which I attributed to condensation/scaly rust. After hitting the brake a few times it would even out. That was a few months ago. Then four days ago, it became a constant problem.

The car only has about 10k miles since restoration. It has a SIII type boosted master cylinder with integral reservoir, and everything in the brake system is basically new.

On inspection of the brakes:

-No leaks/oil contamination.
-Even brake wear, lots of lining remaining.
-Correctly assembled shoes/springs.
-Quality wheel cylinders (Girling).
-Cylinders move freely, no apparent binding.
-Drums are intact, maybe a slight ovalization, not bad.
-Springs are strong and intact.
-Snail cams and steady posts intact, working fine.


Both brakes were disassembled and re-greased, reassembled and adjusted.
In fact, each front brake has been adjusted at least ten times in the last two days. I even slackened the grabbing front right brake a little more and left the front left a little "tighter" to see if it would even out... no effect.

I replaced the front left flex hose on the chance it was collapsing or narrowed internally. Another LR parts vendor is within a short drive of me, and they mentioned they'd seen some bad flex hoses recently. That did not cure the problem though.

Finally, I swapped the drums to see if the one on the right was the cause... no difference.

I haven't observed cylinder travel (right vs. left) because I haven't had a helper... but when I push them by hand they move freely with some resistance on retraction, which is normal.

I am about ready to pull all four front wheel cylinders, rebuild/replace them, purge the whole system, and swap the shoes... but before I do that I would love to hear any suggestions. I cant think how the master cylinder would cause this... I keep coming back to a wheel cylinder or line obstruction.

What do you guys think?

[jp-]

First, has this been a problem since the rebuild? In other words, did they ever work right with the current master cylinder, lines, and wheel cylinders?

If this problem started immediately after the entire brake rebuild, I would suspect that one of the lines routed to the front left (or right) wheel is incorrectly plumbed back to the master cylinder. If you changed over to a dual master cylinder, you must make sure that the t-fitting junction has been removed from the system, or that you don't have a line that should be going to the rear connected to the front or vice-verse.

Next, to check for a line obstruction, just bleed the front brakes and make sure that you are getting adequate flow and pressure from each side.

Lastly, if all of the above check out, make sure that you do not have a rear brake wheel cylinder in place of the correct larger front wheel cylinder. That too could cause the problem.

[jp-]

Another possibility is that you ended up with brake shoes that are just slightly different enough in composition to allow grabbing. Try swapping the shoes. Also, make sure that you don't have two leading shoes on one wheel and two trailing shoes on the other.

[Momo]

Hi jp, this is a new problem and not connected to the restoration, which was done by the previous owner with lots of help from a professional, reputable Series mechanic/restorer. Since I've owned it, it's been all over on trips, city driving, offroad, etc.

It's been totally reliable until the front right started grabbing (just with the first one or two pedal applications) over the summer and now it's all the time.

I have a power bleeder and it seems to be bleeding just fine.

The wheel cylinders are all the correct parts, as are the shoes... 109s have "twin leading shoe" front brakes and the shoes are identical.

[Sputnicker]

I replaced the front left flex hose on the chance it was collapsing or narrowed internally.

Why did you replace the left front flex hose if the right side is locking up? Your symptoms are consistent with a bad hose acting as a check valve. With pressure already in the wheel cylinder(s) and the shoes already riding on that drum, the next time you step on the pedal the pressure locks up that wheel. If it's not the hose, I would suspect a wheel cylinder not retracting. Could be some corrosion in the bore or a seal going bad. If you inspect the bores and they look okay, try a rebuild kit. Lastly, weak brake springs may not be retracting the shoes properly - but I've never experienced that one.

[Momo]

Why did you replace the left front flex hose if the right side is locking up?

Good question! The thought was that if the line had an internal defect that was obstructing flow to that wheel, then the brake response would be weak on that side. In other words, weak on the left and normal on the right = grabbing on the right. But I like your check valve theory, makes much more sense!

I've never seen a weak spring either... they usually just break.

I will follow your theory and see if that's the cause. Easy enough to find out...thanks for the help.

[garycox]

I have seen flex hoses with the check valve type of problem. Externally they look fine, but if you take them off and blow a little air through the internal obstruction is usually obvious.

Application of the brake generates enough pressure within the flex line to force the obstruction back against the sidewall, but the resting pressure within the hose in the reverse direction is not enough to allow the wheel cylinder to bleed off adequately.

[jp-]

Looks like I was off on that one. A bad hose on the right side does make sense. Perhaps it could easily be checked by jacking up both front wheels and applying the brakes, then releasing. If the right one doesn't turn, you have likely found your problem.

[Momo]

Gary, that does indeed sound logical. It would also explain how the problem was intermittent some time ago, subsided after a few pedal applications, and is now constant... if there's a portion of hose that's gradually been delaminating and finally has done so enough to cause the symptoms.

Jp, I'll do just what you suggest tomorrow morning... I hope this is the cause, since it's an easy fix.

Thanks again guys, I'll post an update after I get my hands dirty:D

[Momo]

Well guys, I spent some time on Wednesday trying to track down the source of the problem again. Full inspection of all parts in both front brakes. Everything looked normal with the exception of the wheel cylinder bores, which all have slight scuffing. However, none of the pistons were binding with gentle application of compressed air and all retracted into their bores easily with finger pressure. The right front brake hose seems fine too, but I'll do a postmortem incision to see if there's any defect inside it.

The system is of course drained of fluid and I'll be blowing air through all the pipes in case a bit of crud had made its way into the right front pipe. I'm going to replace the cylinders with more robust Lockheed units, although they are quite serviceable. I'll put them on the shelf as spares... they have less than 10k miles of use.

Thanks again for the help... maybe I'll post some pics later. Happy New Year.

[Momo]

Okay guys, I fixed the problem. In the end it was... the shoes! The edges were grabbing. I know for sure because I swapped them to the left side and the truck started pulling left on brake application.

Why they started grabbing after 10k miles, who knows... maybe just the way they bedded in on that brake. I chamfered the edges and it now stops in a straight line. I ended up replacing the hoses and wheel cylinders anyway, but the old parts are good for spares. Thanks for all your suggestions on this one... Happy New Year!