Can only lock left front wheel on pavement. I'm stumped...

417 Views 18 Replies 10 Participants Last post by DonP, 11 d ago

JohnnyK

Discussion Starter · 12 d ago (Edited)

Well guys, I am still pretty stumped here. Generally, with moderate pressure (on purpose), only the front left wheel will lock up. I have done some exhaustive troubleshooting and parts replacement. Any thoughts? (please read the post if you are inclined to respond, thanks).

Running stock 1968 discs on the front. SN95 discs on the rear.

Rear proportioning valve (SSBC if I recall) is actually all the way open (clockwise screwed in). I didn't realize I had it fully open until I went to open it some more.

1994 Cobra master cylinder. 15/16 bore. Don't seem to be running out of travel. Have many, many more inches to go before it seems anything will make contact (with the physical pedal, at least). Going back a decade here, but I don't recall having luck (or good feel?) with a 1" bore, hence why I dropped 1/16. I seem to remember wanting more travel.

Rotors were new when I built the car (5000mi ago)

Just replaced RH caliper and brake hose (I assumed that was the issue as that was all that is left)

Tried many different pads. Don't remember what I started with. Ran Hawk HPS for awhile. Just threw on some "Dynamic Friction 5000 metallic" pads last week as an experiment.

Power pedal. Correct location.

Stock booster (I bought new from Napa I think, hard to recall?). Spacer is on there

Booster pushrod is adjusted as per what the spec is (can't recall spec)

Vacuum is only 12-13inhg (B-cam). Which is not great I suppose, but, front left does lock.

Brake pressures are (with moderate/strong food pressure, car running):
LF: 1100 psi
LR: 1050
Rears: 800ish

Good full flow from all bleeders/hoses when opened/remove.

Pedal feel is hard to describe, as I have no other vintage mustang to compare. Not much resistance for a couple inches, then firm.

Pedal sinks when car is started. Booster also holds a vacuum. Booster hose not collapsing.

Edit: After a few hard stops, both rotors are almost exactly 116 C/240F.

Once you warm up the brakes, it does tend to pull to the left under very heavy braking. So even something were happening (say, running out of mc travel), it's still braking harder on the left side.

Any thoughts? Now, I haven't beaten the **** out of it on gravel or a wet road, so maybe it is possible to lock both under that scenario, but, doesn't seem too easy on normal dry pavement!

Thanks!

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pollock

· #2 · 12 d ago

Maybe your RH wheel camber is close to 0° and your LH wheel has a little camber, in either direction. Meaning your LH contact patch is smaller.

Or your tire wear is not the same RH/LH.

HPjunki

· #3 · 12 d ago

First off, and I may be wrong, but I believe you need to turn the proportioning valve ccw to open. I think you are only using the fronts. Again, not positive, but does jive with ur issue.
Second, I had a similar issue with pushing the pedal and nothing until, bam! It applies way too much brakes. Fought this for a very long time. In fact ever since a service station did a brake job in the mid 79s. Was never the same since. Turns out, when they rebuilt my booster, they installed an incorrect rod. They misplaced the original. While restoring the car last year, I found this out and couldn’t get the rod short enough with its adjustment and had to cut about 1/4” off. Absolutely perfect since. I know u said it was at the correct length, but it might be worth another look.
Other than that, u might have a slight bit of air in the right front line That is casing the left to hit before the right.
As for the rears, does yours have the inline check valve at the rear axel? Could it be plugged?
About all I got! I certainly understand how frustrating brake issues are on a super simple setup!
Good luck,
Troy
Btw, I have a 67 with power Ford discs front and drums rear.

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JohnnyK

Discussion Starter · #4 · 12 d ago

Could be alignment I suppose, but sure feels like the physical brakes (feels like it gets grippier and grippier/brakes work better the more I use it on the lh side).

HP, no I think it's like a diaphragm regulator where clockwise opens it up. It's definitely not nothing and then too much. It's still very much modulated, just doesn't get as hard/lockup as I would like.

I wonder if just the low vacuum/SLIGHTLY less psi at the RH caliper is causing it? Seems like a stretch but I'm running out of ideas.

Also I would have thought a fully open prop valve would allow the rears to lock on pavement?

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74Rallye

· #5 · 12 d ago

How old are the flexible lines that go to the calipers?

JohnnyK

Discussion Starter · #6 · 12 d ago

JohnnyK said:
Just replaced RH caliper and brake hose (I assumed that was the issue as that was all that is left)

Pittbull

· #7 · 12 d ago

Not all tires are created equal or made the same time, especially in the US where tire regulations are more lax. When a tire gets old it gets hard, this sounds like it wouldn't make a big difference but you'd be surprised at how much difference it does make. Not saying this may be the issue but it is a possibility if one wheel locks up before the other. Tires that have outlived their usefulness on a shelf will not only be harder than a new one, they will be uneven in viscosity, there will be a higher oil content at the bottom as the tire sat on the shelf. I've had motorcycle tires that had thelis issue. Have also gone to buy car tires "on sale", usually a too good to be true scenario and they were noticeably darker on the bottom. Sure enough they were expired.
I only put this out there as a remote possiblity, most likely yours are a set bought and made in a specified timeline.
Most likely your issue is a tiny air bubble hidden inside the slow calipe, it is also further from your master than the one locking up. Could also be the brake line flexible extension is slightly worn that could be relieving the caliper of some pressure.

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JohnnyK

Discussion Starter · #8 · 12 d ago

Thanks guys. I'll take it for another boot tonight and see if these new pads don't improve anything (I have my doubts after 5000 miles of experimenting though).

rpm

· #9 · 12 d ago

1,100 pounds of line pressure seems low to me.

Woodchuck

· #10 · 12 d ago

A couple thoughts......
a. Strut rod bushing condition.
b. Loose wheel bearings.
c. Binding caliper slides.

Reactions: 1

JohnnyK

Discussion Starter · #11 · 12 d ago

Seems a little low to me too, but I've never measured any other car!

Bart, rod end adjustable strut rods, tight. Wheel bearings tight, calipers newly removed and reinstalled (when I replaced the RH caliper and did the pads last week. Maybe LCA or sway bar?? But, I scuffed the rotors as well so tomorrow I'll be able to see if everything is wearing properly.

So I went out for a drive and did 3 panic stops (well, I did a lot more, but only photographed 3 of them). So I can lock them up, but they lock later.. And the second time, looks like it did not lock. (I think locking the rears is definitely out of the question though). I mean, maybe the 2nd stop the rh would have locked if i pushed more, but, still something seems fishy to me. Shrug?

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HPjunki

· #12 · 12 d ago

JohnnyK said:
Could be alignment I suppose, but sure feels like the physical brakes (feels like it gets grippier and grippier/brakes work better the more I use it on the lh side).

HP, no I think it's like a diaphragm regulator where clockwise opens it up. It's definitely not nothing and then too much. It's still very much modulated, just doesn't get as hard/lockup as I would like.

I wonder if just the low vacuum/SLIGHTLY less psi at the RH caliper is causing it? Seems like a stretch but I'm running out of ideas.

Also I would have thought a fully open prop valve would allow the rears to lock on pavement?

View attachment 897072

Guess it depends on if “increase” means increase the front brakes, or not. Might be worth turning the other way a bit. Seems suspicious since the fronts only lock.
I still would check for any air in the right front line.
Troy

Woodchuck

· #13 · 12 d ago

JohnnyK said:
Seems a little low to me too, but I've never measured any other car!

Bart, rod end adjustable strut rods, tight. Wheel bearings tight, calipers newly removed and reinstalled (when I replaced the RH caliper and did the pads last week. Maybe LCA or sway bar?? But, I scuffed the rotors as well so tomorrow I'll be able to see if everything is wearing properly.

So I went out for a drive and did 3 panic stops (well, I did a lot more, but only photographed 3 of them). So I can lock them up, but they lock later.. And the second time, looks like it did not lock. (I think locking the rears is definitely out of the question though). I mean, maybe the 2nd stop the rh would have locked if i pushed more, but, still something seems fishy to me. Shrug?

View attachment 897084
View attachment 897083
View attachment 897086

I'd give the caliper slides and leaf springs a good hard look... the mounts, too.

JohnnyK

Discussion Starter · #14 · 12 d ago

HPjunki said:
Might be worth turning the other way a bit.

You know, a smarter man may have checked the proper operation of that valve when he had a gauge attached to the rear calipers..

D10E

· #15 · 12 d ago

swap flexible brake lines side to side and see if it follows the hoses. Heck they're cheap enough to just replace for good measure. I've had one swell pretty noticeably on brake apply and the car would pull to the opposite side on a hard brake apply.

HPjunki

· #16 · 11 d ago

JohnnyK said:
You know, a smarter man may have checked the proper operation of that valve when he had a gauge attached to the rear calipers..

Lol! We have all been there. And will be there again.
Troy

Reactions: 1

MT

· #17 · 11 d ago

D10E said:
swap flexible brake lines side to side and see if it follows the hoses. Heck they're cheap enough to just replace for good measure. I've had one swell pretty noticeably on brake apply and the car would pull to the opposite side on a hard brake apply.

I agree with this. I had the same problem with my truck. It turned out that the lining of one of the hoses had a hole in it and fluid pressure to the calipers would be delayed on quick application of the brakes. It would even out after a couple seconds. Skid marks looked the same as yours.

JohnnyK

Discussion Starter · #18 · 11 d ago

Guys, I appreciate the advice but read over the post again (hoses not the issue unfortunately).

DonP

· #19 · 11 d ago

I'd start swapping hardware from one side to the other. Tires first because they're easy, then pads and rotors.

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