Re: Brake Pressure Question
[Re: justinp61]
#2469836
03/21/18 05:53 PM
03/21/18 05:53 PM
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Joined: Jun 2003
Posts: 52,972 Romeo MI
MR_P_BODY
Master
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Master
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Posts: 52,972
Romeo MI
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Andy, I have the fitting to go in the line to remove the adjustable proportioning valve and will most likely get it changed today. I'll go over the lines to see if there are any kinks, it has 3AN flex lines. When I get some help I'll check the pressure again.
CMcAllister, the fronts are Strange race brakes I bought from Hemi Fred years ago, the only other option I see for calipers on these brakes are 1.75" pistons. I can go to a larger piston on the rear Wilwood's if needed but their tech didn't think I needed to. Once I get the pressure figured out I'll see how it stops and make any needed changes.
Thanks guys, this brake stuff is greek to me. lol
Even if a line is kinked but not totally closed off that pressure will finally equal out on that line..in a period of time.. same thing as a prop valve... this just hyd 101
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Re: Brake Pressure Question
[Re: MR_P_BODY]
#2469873
03/21/18 06:52 PM
03/21/18 06:52 PM
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Joined: Oct 2003
Posts: 11,686 W. Kentucky
justinp61
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W. Kentucky
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Even if a line is kinked but not totally closed off that pressure will finally equal out on that line..in a period of time.. same thing as a prop valve... this just hyd 101 Understood, the proportioning works an adjustable orifice restricting flow, thus initially pressure too. In a closed system like brakes the pressure will eventually equalize on both sides of the orifice. Right now I just want to find why my pressure isn't where it needs to be on the initial pedal pump. The proportioning valve is out and the union is in. Have to work tonight so hopefully tomorrow I'll re bleed the fronts and see what I have. Thanks, Justin.
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Re: Brake Pressure Question
[Re: CMcAllister]
#2469994
03/21/18 09:06 PM
03/21/18 09:06 PM
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Joined: Jan 2003
Posts: 26,005 Rio Linda, CA
John_Kunkel
Too Many Posts
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Posts: 26,005
Rio Linda, CA
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Consider 500psi of fluid pressure at one 4-piston caliper.
With 1.38 pistons, the force exerted on the pads is 2960lb. total. With 1.75 pistons, the force exerted on the pads is 4800lb. total.
It would seem that way but it isn't; when computing the clamping force of multi-piston rigid calipers you only use half the caliper pistons. IOW, only one side applies force, the other side acts as a backup. I know this is counterintuitive but that's the way it is.
The INTERNET, the MISinformation superhighway
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Re: Brake Pressure Question
[Re: John_Kunkel]
#2470000
03/21/18 09:20 PM
03/21/18 09:20 PM
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Joined: Dec 2005
Posts: 11,739 Fulton County, PA
CMcAllister
Mr. Helpful
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Mr. Helpful
Joined: Dec 2005
Posts: 11,739
Fulton County, PA
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Consider 500psi of fluid pressure at one 4-piston caliper.
With 1.38 pistons, the force exerted on the pads is 2960lb. total. With 1.75 pistons, the force exerted on the pads is 4800lb. total.
It would seem that way but it isn't; when computing the clamping force of multi-piston rigid calipers you only use half the caliper pistons. IOW, only one side applies force, the other side acts as a backup. I know this is counterintuitive but that's the way it is. After re-reading the tech information I was using, I believe that is correct. I had just skimmed through it earlier. The numbers can be divided in half. But the percentage of difference between using the 1.38, instead of the typical larger bore units, is still valid.
If the results don't match the theory, change the theory.
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Re: Brake Pressure Question
[Re: cudaman1969]
#2470435
03/22/18 03:32 PM
03/22/18 03:32 PM
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Joined: Jun 2003
Posts: 52,972 Romeo MI
MR_P_BODY
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Master
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Posts: 52,972
Romeo MI
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I don't think I would want more stopping on the back, street or strip. Once it starts sliding the car tends to swap ends. Did you bleed that MC good? I would start the guage at the MC and work down. On a drag car with big rear tires you want more breaking in the rear.. if you have more breaking in the front it will try to swap ends.. a street car the breaking is 70/30 most on the front.. but the tires need to be matched or close.. the tires do the work..also if you run skinny fronts they will try to lock up and skid(possible blow outs)
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Re: Brake Pressure Question
[Re: justinp61]
#2470471
03/22/18 04:39 PM
03/22/18 04:39 PM
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Joined: Jan 2005
Posts: 2,540 Milwaukee WI
TRENDZ
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master
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Milwaukee WI
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Without knowing weight bias, you can’t really say it should be either way. I can tell you that my own car, that had countless hours of experimenting on the brake system, was as well balanced (brake wise) as any car I had ever driven. It was a mish mash of odd parts that worked in orchestra on a heavy car that had no business going as fast, or stopping as quick as it did. Each front wheel (26” tall Eagle front runner) had 1100 pounds static weight, while the rear (31” 10.5W MT) were at 750 pounds static weight. The car had 12”dia vented front rotors with wilwood large piston calipers. The rear of the car had 10” drum brakes from a cuda or some such car. The master cylinder was what you would find in a late 80s caravan. Those master cylinders have no built in bias because they are designed for a cross braking application. My adjustable prop valve was in the rear brake circuit. I adjusted out some rear brake until i could skid the car evenly at speed.
"use it 'till it breaks, replace as needed"
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Re: Brake Pressure Question
[Re: TRENDZ]
#2470483
03/22/18 04:58 PM
03/22/18 04:58 PM
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Joined: Jun 2003
Posts: 52,972 Romeo MI
MR_P_BODY
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Master
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Romeo MI
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Without knowing weight bias, you can’t really say it should be either way. I can tell you that my own car, that had countless hours of experimenting on the brake system, was as well balanced (brake wise) as any car I had ever driven. It was a mish mash of odd parts that worked in orchestra on a heavy car that had no business going as fast, or stopping as quick as it did. Each front wheel (26” tall Eagle front runner) had 1100 pounds static weight, while the rear (31” 10.5W MT) were at 750 pounds static weight. The car had 12”dia vented front rotors with wilwood large piston calipers. The rear of the car had 10” drum brakes from a cuda or some such car. The master cylinder was what you would find in a late 80s caravan. Those master cylinders have no built in bias because they are designed for a cross braking application. My adjustable prop valve was in the rear brake circuit. I adjusted out some rear brake until i could skid the car evenly at speed. On my P-body I had front runners and 31x14 the adj prop was on the front.. master was a 3/4 mopar unit.. I could lock all 4 tires if I had to but only did that testing the prop till it was set right(disc on all four)
Last edited by MR_P_BODY; 03/22/18 05:08 PM.
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Re: Brake Pressure Question
[Re: krautrock]
#2470561
03/22/18 06:49 PM
03/22/18 06:49 PM
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Joined: Jun 2003
Posts: 52,972 Romeo MI
MR_P_BODY
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Master
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Posts: 52,972
Romeo MI
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I was under the impression that the aluminum master cylinders that are popular on our cars don't have any bias built into them, but they do have two different volumes for each side of the reservoir which is why the rear port is supposed to go to the larger volume system on the car. Not all alum master are biased.. my 3/4" master isnt but a lot of larger masters are.. you would need to check what you have.. mine has the larger side also.. I have a couple of alum masters that are the same size.. I got a FEW masters when I was working in the brake lab( I was on loan to the brake lab but my room mate worked there all the time)
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Re: Brake Pressure Question
[Re: justinp61]
#2470706
03/22/18 10:03 PM
03/22/18 10:03 PM
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Joined: Jan 2005
Posts: 2,540 Milwaukee WI
TRENDZ
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Milwaukee WI
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Even if a line is kinked but not totally closed off that pressure will finally equal out on that line..in a period of time.. same thing as a prop valve... this just hyd 101 Understood, the proportioning works an adjustable orifice restricting flow, thus initially pressure too. In a closed system like brakes the pressure will eventually equalize on both sides of the orifice. Right now I just want to find why my pressure isn't where it needs to be on the initial pedal pump. The proportioning valve is out and the union is in. Have to work tonight so hopefully tomorrow I'll re bleed the fronts and see what I have. Thanks, Justin. I have never seen an aftermarket proportioning valve that worked or functioned this way. Every adjustable prop valve I’ve ever come across is an accumulator/spring that can only reduce pressure... not flow. This is why opening / unscrewing the valve decreases pressure (less spring load absorbs more fluid into the small accumulator. I guess technically flow is slightly reduced by the fact that a vey small amount of fluid is absorbed in the accumulator, but there is no flow orfice. Also, pressure will not equalize using any of the adjustable proportioning valves I’ve used. Can anyone show me one of these flow style valves? Maybe I’m missing something.
"use it 'till it breaks, replace as needed"
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Re: Brake Pressure Question
[Re: SportF]
#2470750
03/23/18 12:09 AM
03/23/18 12:09 AM
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Joined: Jan 2004
Posts: 8,356 fredericksburg,va
cudaman1969
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master
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Posts: 8,356
fredericksburg,va
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When you say a master cylinder is biased, are you saying you get different pressure out of one line VS another? Is that possible?
I don't think it is.
When a car is designed, the size of pistons front or back gives the manufacturer the calculated brake capabilities.
And that factory valve that most call a proportioning valve is not that at all. It is a shuttle valve that gives you the dual/failure mode braking if front or back leak/fail. This is not done in the master cylinder.
I've been wrong before, so I am correctable, eh? Finally someone got it right, thank you. unbelievable the amount of wrong info.
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Re: Brake Pressure Question
[Re: justinp61]
#2470752
03/23/18 12:10 AM
03/23/18 12:10 AM
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Joined: Jan 2005
Posts: 2,540 Milwaukee WI
TRENDZ
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master
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Milwaukee WI
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Yes you can get different pressures out of the master cylinder. I understand your query. Since the bore is the same, it would seem that output would be the same. The way you can get a pressure bias is through piston surface area. The rear most piston has a centered semi floating rod that deducts from the overall surface area of that piston. The second piston has no rod, or less of a rod. More surface area= less pressure.
"use it 'till it breaks, replace as needed"
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Re: Brake Pressure Question
[Re: TRENDZ]
#2470760
03/23/18 12:20 AM
03/23/18 12:20 AM
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Joined: Jan 2004
Posts: 8,356 fredericksburg,va
cudaman1969
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master
Joined: Jan 2004
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fredericksburg,va
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Yes you can get different pressures out of the master cylinder. I understand your query. Since the bore is the same, it would seem that output would be the same. The way you can get a pressure bias is through piston surface area. The rear most piston has a centered semi floating rod that deducts from the overall surface area of that piston. The second piston has no rod, or less of a rod. More surface area= less pressure. If the bore is the same for both piston cups there will be no difference. A rod pushes the rear cup and fluid pressure alone pushes the from cup to make up for clearance issues. Has anyone ever taken one of these apart to see how they work?! Or is this something you read off the net.
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Re: Brake Pressure Question
[Re: justinp61]
#2470768
03/23/18 12:34 AM
03/23/18 12:34 AM
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Joined: Aug 2011
Posts: 2,872 Pattison Texas
CSK
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master
Joined: Aug 2011
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Pattison Texas
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There are step bore master cylinders also, they have different pressure & volume on the ports.
1968 Charger COLD A/C Hilborn EFI 512ci 9.7 compression, Stealth heads, 4.10 gear A518 ODtrans 4100lb,10.93 full street car trim 2020 T/A 392 Stock 11.79 @ 114.5
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Re: Brake Pressure Question
[Re: CSK]
#2470790
03/23/18 01:34 AM
03/23/18 01:34 AM
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Joined: Jun 2003
Posts: 52,972 Romeo MI
MR_P_BODY
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Master
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Posts: 52,972
Romeo MI
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Prop valve What does a proportioning valve do? A proportioning valve is used in the rear to decrease the rate of pressure rise to the drums relative to the pedal force as weight is shifted to the front during braking. This prevents the rear from locking up under hard braking conditions. The set up above is for a car with equal sized tires
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