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four wheel discs, prop valve in front circuit? |
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tomj 
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Joined: Jan/27/2010 Location: earth Status: Offline Points: 7539 |
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 Topic: four wheel discs, prop valve in front circuit?Posted: Mar/26/2017 at 12:47am |
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i recently switched to discs on all four wheels ('98 mustang rear axle, Scarebird kit in front). my front/rear balance is bad, the rears don't grab enough. 1" dual circuit master cylinder, no power assist. when i had drums on the rear (old AMC axle) i juggled rear wheel cylinders to get it balanced. not a choice here.
there are no mechanical or other problems with the brakes, master, calipers, lines, etc. all new parts, etc. working great, except for balance.
two questions: 1) i have an adjustable prop valve plumbed into the FRONT circuit. haven't driven the car yet. anyone see a problem with a Summit knob-adjustable proportioning valve in the FRONT brake circuit? 2) anyone see any reason why either port on the master cylinder can't go arbitrarily to the front or the rear circuit? both are the same bore, resevoir and stroke fluid capacity is adequate. i don't see a reason other than convention for one port over the other. is there a reason i don't know of? |
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1960 Rambler Super two-door wagon, OHV auto
1961 Roadster American, 195.6 OHV, T5 http://www.ramblerLore.com |
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Red Devil
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Joined: Jul/10/2007 Location: Canada Status: Offline Points: 1743 |
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Posted: Mar/26/2017 at 9:57am |
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1) If the front circuit is for your rear brakes, then that's where you would put the proportioning valve. If front circuit is for front brakes, then the proportioning valve goes in rear circuit ... assuming it's a typical road car braking system with normal front/rear bias. 2) Most tandem masters have a 2:1 or similar volume ratio where one circuit delivers twice the volume of the other. In a normal balanced braking system, the higher volume circuit goes to the front. Some masters have the high-volume circuit to the front port and some the rear port, so need to check your master and plumb to suit. If your rear brakes aren't working well, check pressure front/rear, check that calipers are working (if they are type with integral park brake, likely need adjusted), calculate the bias based on piston sizes and upgrade the rear calipers or use pads with higher CoF or larger diameter rotors to get more rear braking or reduce front caliper piston size or use lower CoF pads, reduce pedal ratio or fit better tyres that take advantage of your braking torque if fronts lock too easily, then match up the rears to suit. A proportioning valve is primarily to limit rear pressure rise under max braking conditions when weight transfer to the front is highest - not to fix an inherent bias issue. if you don't want to change calipers, pads, etc. a better solution may be to fit dual master cylinders with a balance bar where you can use different master cylinder bore sizes to regain balance and tune with the balance bar. Hope this helps,RD
Edited by Red Devil - Mar/26/2017 at 11:39am |
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tomj
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Joined: Jan/27/2010 Location: earth Status: Offline Points: 7539 |
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Posted: Mar/27/2017 at 12:14am |
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hmm i get your point(s) RD. yeah, i have inherently mis-matched parts, inevitable under the circumstances. everything's working right, the problem is dissimilar piston areas in the calipers. the rears actuate OK, just not enough. 98 mustang 7.5" axle, 10" or 11" whatever discs.
it's not deadly or anything. the park brake works great (with the '61 parking cable system as-is, pleasantly enough). probably the first thing to try is to get some nice soft grippy pads for the rear. cheap and easy. my master cyl is in the floor, a chevy type with a flat steel cover bolt on. would require complete reengineer to put two masters with a balance bar. id ;likely just put up with bad balance if that's the only option :-) it would have been better if i'd done that in the first place. i admit i haven't done the math on piston areas. i will do so. thanks! |
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1960 Rambler Super two-door wagon, OHV auto
1961 Roadster American, 195.6 OHV, T5 http://www.ramblerLore.com |
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farna
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Moderator Lost Dealership Project Joined: Jul/08/2007 Location: South Carolina Status: Offline Points: 19672 |
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Posted: Mar/27/2017 at 7:02am |
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I'm assuming you have no prop. valve in the rear at all? If you do, remove it. That might fix the issue, or make it better. The other thing to try is swapping the output lines. Last thing -- are you sure you can't find different sized wheel cylinders for the Ford rear axle? It was used in a lot of different cars -- I'd think there should be some different sized wheel cylinders, albeit a limited number. Four cylinder vs. V6 model Rangers? Maybe Pinto/Mustang II wheel cylinders, of four cylinder Fox body? Not as easy as the older Bendix... or did Ford continue to use Bendix?
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Frank Swygert
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Lyle
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Posted: Mar/27/2017 at 2:46pm |
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There are several adjustable proportioning valves at Summit. When I did my disc rears research indicated a valve with inputs from both master cylinder outputs did a better job for balancing then just a single one line unit as the master cylinders may have designed specific output pressure/volume front and rear. If Farna's suggestion above does not work you may look at a better proportioning valve setup.
This is the unit I used: https://www.summitracing.com/int/parts/ssb-a0730 Set for 20% and works very well.
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tomj
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Joined: Jan/27/2010 Location: earth Status: Offline Points: 7539 |
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Posted: Mar/27/2017 at 11:47pm |
disc brakes in the rear, not drums. drums are easy! i did have a prop valve in the rear, from when i had the AMC axle with drums out back. i left it in and backed it out all the way, should be wide open. today i looked at the mustang pads i got -- they're ceramic. and don't fill out the backing (smaller area). so i think the first, easiest, is to call someone reputable and get soft organic pads, remove the prop valve from the front (just plumbed it, car still on stands) and start from there i think. |
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1960 Rambler Super two-door wagon, OHV auto
1961 Roadster American, 195.6 OHV, T5 http://www.ramblerLore.com |
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tomj
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Joined: Jan/27/2010 Location: earth Status: Offline Points: 7539 |
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Posted: Mar/27/2017 at 11:51pm |
oh! right! OK. in my shopping i missed that, it is specially designed for balancing. i printed out the page, i will revisit this once i get the @#$@_*% blasted engine in and car on the road and see if the pads help. thanks for that!!
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1960 Rambler Super two-door wagon, OHV auto
1961 Roadster American, 195.6 OHV, T5 http://www.ramblerLore.com |
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farna
Supporter of TheAMCForum
Moderator Lost Dealership Project Joined: Jul/08/2007 Location: South Carolina Status: Offline Points: 19672 |
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Posted: Mar/28/2017 at 7:37am |
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The SSBC valve in the link above may be a better overall valve, but the front brake line doesn't do anything but split the front brakes like a T. The pressure brake switch I don't care much for, but maybe they use a better one than you can get from most parts places (Ron Francis Wiring gets a good one from somewhere, but standard NAPA and others won't last long). With the price I'd at least hope it's just a better quality valve.
Typically, four wheel disc brake setups don't use a proportioning valve at all. I don't have one and am using 79 Spirit front brakes (2.6" single piston) and Jaguar rear brakes (opposed dual piston... don't recall the diameter, but I think under 2"). I haven't experienced any rear wheel lockup or other issues, but then my 63 Classic wagon is a lot heavier than Tom's little roadster. I think Tom's on to something with the brake pad composition. Sounds like the rear pads may be much stickier than the fronts. So softer rear pads may be the key -- or much better front pads so they all four lock at the same time -- which probably isn't a good situation! Locked means sliding straight, would be harder to control braking if they all lock too easily, especially in spirited driving! |
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Frank Swygert
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Red Devil
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Posted: Mar/28/2017 at 8:51am |
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Most parts store replacement pads will have similar CoF as stock as they are designed to work as stock replacements with similar performance. Biggest differences are in temperature range, dusting, noise and longevity. To get a higher CoF pad, typically have to go to aftermarket performance pads from Hawk, EBC, PFC, Wilwood, StopTech, etc.
Personally, I wouldn't change any parts until you calculate the bias you have now. No sense throwing money at it until you know the starting point ... and typically best to calculate the bias before making any changes from stock as it's tougher to fix after parts are installed. Don't know the caliper piston size of the Scarebird kits, but assume close to stock piston size? Expect the Mustang used smaller front caliper pistons to match the smaller rears with either a smaller bore master or more boost assist. Hope this helps,RD. |
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Lyle
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Posted: Mar/28/2017 at 10:24am |
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As usual Frank is correct that the valve I posted is just a "T" for the front brakes but it does use the pressure bias to correct/adjust for the rear brakes. I installed a shortened Ford Explorer 8.8 disk brake posi as they are cheap here, but the caliper pistons are 1-7/8" 10.5" disc. Way too much brake for the rear .
I did calculate the bias but this valve just made it too easy to not do it. Also was able to keep the original differential switch "as is" between the master cylinder and proportioning valve per original design. Can go back if I ever want to, but is stops better then any of my "modern" vehicles.
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farna wrote: