So I put some new brake pads on the Jeep yesterday. And now I have the same pads front and rear (Carbotech AX6s) instead of having weaker pads in the rear.
And I found a problem... The stock brake proportioning was based on the weigh transfer at max braking with 1998 vintage 225 all seasons, not 240 treadwear 255s. So the rear brakes lock up way too early on dry pavement when you get on the pedal hard.
The fix for this is obviously an adjustable prop valve. But here's the question: Is there anything wrong with adding an adjustable prop valve into the rear line output from the stock combination valve? Or do I need to replace the whole thing with an adjustable combination valve (more $$$)?
In addition, I'll have to see how it feels once I fix the proportioning, but I think I might need to swap to a slightly bigger master cylinder now... Stock is a 1" bore, I'm thinking I might want to go to a 1 1/16" bore, as the pedal effort to achieve lockup is a bit too low for a non-ABS setup IMO with the new pads (even dead cold, they stop like hitting a brick wall if you lean into the pedal a bit).
And as a review note on the AX6s: After a thorough bed-in, they're definitely quiet enough to use as DD pads (they squeak a bit sometimes under light braking at low speeds, but nothing awful). However, they dust. A lot. As in, stock BMW pads don't dust at all in comparison to these. I cleaned the wheels off after bedding the pads in (they were almost black). I drove the thing about 12 miles after that and already had visible dust again...
I'd prefer just a single prop valve in the system so you don't get weird interactions.
Ive done it both ways. Obviously single adjustable is preferable.
You can leave the stock block, just bypass it with a hardline from output of master directly to input of adjustable.
Are the rotors, calipers, etc all stock? Is it 4-wheel disc, or are the rears drums?
If stock and discs, I'm surprised that the factory braking system was sending enough to the rear that a shift to stickier street tires would push it over to rear bias, I would have expected it to have a large-enough built-in front bias that it could accommodate this. I've managed that on my Miata, but it took R compound track tires to do it, as well as significant other changes to the calipers/etc.
Have you read this article on prop valves? http://stoptech.com/technical-support/technical-white-papers/proportioning-valves
I have that stoptech article open right now!
As far as the system, it's discs all around, stock calipers, rotor size, etc.
The brake system had plenty of front bias stock, but apparently not enough extra to deal with this much more weight transfer (being tall makes the weight transfer worse than it would be in a Miata or similar).
As an interesting note, I took an IR gun to the rotors after the bed-in run (after about 2 minutes of gentle driving at the end, so things had cooled down a bit). Fronts read about 490 degrees, rears were around 630. So the rears are definitely doing more than their fair share (they also have much worse cooling, rear rotors are solid instead of vented and they get less natural airflow).
Leaving the stock block with 1 side bypassed would be no good, I'd have a permanent brake failure warning on the dash (the uneven input pressure trips the switch in the stock combo valve). Plus, it wouldn't be doing anything at all in that situation.
I'm currently poking through the prop valves available on Summit to see what suitable options I can find.
So at this point, it looks like my options are to either throw one of these in the rear line: http://www.wilwood.com/MasterCylinders/MasterCylinderProd.aspx?itemno=260-10922
Or replace the stock combo valve with one of these (only downsides are more $$$ and losing the brake warning light on the dash, but the light isn't important IMO): http://www.wilwood.com/MasterCylinders/MasterCylinderProd.aspx?itemno=260-15048
You can put the one-in / one-out type adjustable downstream of the OE valve with no ill side-effects.
AngryCorvair wrote:
You can put the one-in / one-out type adjustable downstream of the OE valve with no ill side-effects.
Alright. That was the original plan, and now that I'm looking at some stuff, that might be the less annoying option, as the stock fittings are a mix of metric bubble flares and 3/8-24 inverted flare. They're metric at the master apparently and they're metric at the ABS pump (being removed).
If I leave the stock prop valve in place, I won't need to screw with any of the metric stuff as I can just leave the stock master to prop valve lines in place. This is the easy way out and doesn't hurt my ability to upgrade the master either, as if I decide I need a bigger bore master, I can get a 1 1/16" bore master from a Durango that bolts right in. There's also 1 1/8" and 1 1/4" options from other similar year Mopars, but I'm not sure if those are 100% bolt in (and I doubt I'll need to go that big anyway).
I've run an adjustable valve behind the stock combination valve on a couple of things. I agree that its no problem.
I just spent the money for the combination adjustable prop valve/distribution block. I try not to add extra fittings in my brake system.
Everything I've ever read says that putting two prop valves in series is a really bad idea.
I just ordered the Wilwood distribution block / prop valve combo unit. The bracket they provide should mount right up to my master cylinder and it looks like I'll be able to re-bend and possibly cut and re-flare the stock MC to prop valve lines, so I should be able to avoid needing to screw with the metric fittings on the master cylinder.
After running some errands in the Jeep and paying attention to just how low the pedal effort is for normal to light braking, I ordered a Durango master cylinder as well. If I did the math right, it should give a 12 - 13% increase in pedal effort, so things should feel better once I get a chance to put the new parts in.
