Brake line pressure

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Brake line pressure

Post by Ron Gusack » Mon Apr 23, 2018 12:41 pm

What should the rear line pressure be on a 65 Chevelle with manual drums in the rear and discs up front or where can I find specs like that? My buddy has a pro street Chevelle that has 650 psi at the wheel cylinder and it doesn't stop well at all.
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Re: Brake line pressure

Post by Brian P » Mon Apr 23, 2018 6:21 pm

Does it have a proportioning valve in the rear circuit?

What size are the front brake rotors, what diameter are the front brake caliper pistons, how many pistons on each side of the caliper?

I know you asked about the rear ... but the front brakes do most of the stopping, to the extent that the rear just helps a little.

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Re: Brake line pressure

Post by Geoff2 » Tue Apr 24, 2018 5:49 am

If the discs are only getting 650 psi, throwing an anchor out the window might help.............. because 650 psi isn't.

Two probable causes: m/c bore too large [ for manual braking; probably needs to be 15/16" or 7/8"]. Wrong pedal hole used for m/c pushrod attachment. Should be top hole for manual brakes.

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Re: Brake line pressure

Post by Ron Gusack » Tue Apr 24, 2018 9:43 am

Brian P wrote:
Mon Apr 23, 2018 6:21 pm
Does it have a proportioning valve in the rear circuit?

What size are the front brake rotors, what diameter are the front brake caliper pistons, how many pistons on each side of the caliper?

I know you asked about the rear ... but the front brakes do most of the stopping, to the extent that the rear just helps a little.
It has an aftermarket proportioning valve that's been gradually opened to give max pressure to the rear drums. I'm not sure on the rotor size or the piston size, but I know there's only one piston in the caliper. The front discs are factory 68-72 Nova.
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Re: Brake line pressure

Post by Brian P » Tue Apr 24, 2018 12:40 pm

I would expect 1500 - 2000 psi brake line pressure to be required for a hard stop. Obviously with a manual master cylinder, this will be dependent on how hard someone pushes the pedal, but if it requires beyond any rational human capability to achieve that sort of line pressure to the front brakes then you need more leverage. As mentioned before, this could be either by a smaller piston master cylinder or more leverage between the brake pedal and the master cylinder.

2000 psi for a 15/16" bore master cylinder would take about 1300 lbs pushing on the master cylinder piston rod. I don't know what the leverage ratio is between the pedal and the master cylinder but if it's 5:1 it would take 260 lbs on the pedal to do that ... it will require a hard shove. A smaller bore master cylinder will require less effort but if you go too small, the pedal will hit the floor first, and you certainly don't want that, either. There's a reason that everyone went to power brakes!

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Re: Brake line pressure

Post by Ron Gusack » Tue Apr 24, 2018 1:08 pm

His front brakes work fine. it's the rear drums that do almost nothing. The shop that has the car says the rear cylinders measured 650 psi and they said that's plenty. They told him he needed discs in the rear. My question is how much pressure does a 9" drum brake Chevelle need??
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Re: Brake line pressure

Post by Brian P » Tue Apr 24, 2018 2:24 pm

What's the front brake line pressure under the circumstances when the rear brake line pressure is 650 psi? That will tell the tale on whether the proportioning valve is at fault.

Up to the setpoint of the proportioning valve, rear brake line pressure should equal front brake line pressure with possible slight deviation by a component called a "residual pressure valve" commonly found in drum-brake applications, whose function is to maintain a little bit of brake pressure in the line when the brakes are not applied. The residual pressure should be around 2 psi ... negligible. Not all brake systems have this valve.

When the brake line pressure reaches the setpoint of the proportioning valve, only then does the proportioning valve have any effect at all. It will limit rear brake pressure once the front brake pressure (from the master cylinder) exceeds its setpoint. Front brake line pressure should be unregulated from the master cylinder.

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Re: Brake line pressure

Post by Ron Gusack » Tue Apr 24, 2018 2:51 pm

Brian P wrote:
Tue Apr 24, 2018 2:24 pm
What's the front brake line pressure under the circumstances when the rear brake line pressure is 650 psi? That will tell the tale on whether the proportioning valve is at fault.

Up to the setpoint of the proportioning valve, rear brake line pressure should equal front brake line pressure with possible slight deviation by a component called a "residual pressure valve" commonly found in drum-brake applications, whose function is to maintain a little bit of brake pressure in the line when the brakes are not applied. The residual pressure should be around 2 psi ... negligible. Not all brake systems have this valve.

When the brake line pressure reaches the setpoint of the proportioning valve, only then does the proportioning valve have any effect at all. It will limit rear brake pressure once the front brake pressure (from the master cylinder) exceeds its setpoint. Front brake line pressure should be unregulated from the master cylinder.
I doubt that the shop working on the car took a pressure reading for the front brakes but I'll ask. I always though residual valves were only on disc brakes. The shop that has the car wants to put Wilwood discs on the rear. Thanks for your help.
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Re: Brake line pressure

Post by Brian P » Tue Apr 24, 2018 3:14 pm

Presumably Wilwood disks of larger diameter and larger brake caliper piston area ... and presumably while retaining the existing stock (hardly high-performance!) front brake system (and I'm only assuming that, since nothing has been stated otherwise ... maybe they want to do all four to maintain a balanced configuration ... I can't tell from my end of the internet!).

That's not a good plan, regardless of tire sizes. The rear tires might be substantially wider than fronts (typical on drag cars) but unless there is some enormous deviation from stock layout, it doesn't change that this is a front engine rear axle drive car with the engine centered slightly ahead of the front wheel centerline, i.e. front-biased weight distribution. The front-biased weight distribution means that regardless of tire sizes, you STILL want most of the braking to be done by the front wheels - sufficient to lock the front wheels before the rear wheels. Battery in the trunk isn't emough to counterbalance that enormous engine up front.

In a panic slam-on-the-brakes situation, locked front brakes mean you can no longer turn but it's a stable situation, in that the front of the car stays out front and the car stops more or less straight; if you want to regain steering control all the driver has to do is ease up on braking a little. Locked rear brakes is unstable and will make the car want to go sideways and swap ends.

If they want to upgrade ALL FOUR brakes, maintaining the stronger brakes up front, then I'll stop (and so will the car)! But it would be overkill if it turns out that the real problem is merely a bad or wrongly selected proportioning valve or master cylinder ...

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Re: Brake line pressure

Post by Ron Gusack » Tue Apr 24, 2018 7:49 pm

This is a street driven "prostreet" car with super wide rear tires. The plan is to just do the rear brakes. I think the master cylinder is wrong or the pedal ratio is wrong or maybe both. A mechanic friend of my friend put larger brake lines from the MC to the rear brakes, which I told my buddy wasn't the way to go if the pressure was already low. I'll check in with him and see what the latest plan is.
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Re: Brake line pressure

Post by Geoff2 » Wed Apr 25, 2018 6:06 am

I think you might be confusing the residual line pressure valve [ RLPV ] with the proportioning valve.The RLPV was only used with drum brakes, & was deleted by many car makers. Using RLPVs with discs brakes can cause the discs to drag. Proportioning valves were used when discs arrived on the scene because the discs need higher line pressure than drums, & some method was needed to reduce/proportion the pressure to the drum brakes.

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Re: Brake line pressure

Post by Ron Gusack » Fri Apr 27, 2018 10:18 am

They've ordered Wilwood discs with 4 piston Corvette calipers for the rear and a Wilwood MC for the rear. Hopefully it all works out. Thanks to all for the posts sharing your knowledge.
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Re: Brake line pressure

Post by Ron Gusack » Thu May 24, 2018 4:15 pm

Here's the latest update. Tom, my buddy, picked up the car today and the brakes are MAYBE slightly better after roughly $1400 spent. Wilwood 11" kit for the rear and a 1.125 MC. I sent him a link on how to measure pedal ratio and I'm hoping he finds something there to improve the stopping ability of this car.
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Re: Brake line pressure

Post by FC-Pilot » Tue May 29, 2018 4:13 pm

I just pulled up some data from a few different vehicles I have in front of me and the rears were cycling the ABS between 1100-1300 PSI. Now that is for Disk brake stuff as I don't have any data for drum brake cars (as there are not many being made anymore and we don't consider then our competition). I will look through some more data and see if I can find old info to compare to. Can you put a force transducer on the pedal to get an idea of how hard you are pushing to get that pressure? That would really shed some light on the issue. As a reference point, all FMVSS 135 testing mandates a maximum of 500 N of pedal force. That is 112 Lb. So a panic, full ABS stop should take 112 Lb or less.
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Re: Brake line pressure

Post by Ron Gusack » Tue May 29, 2018 9:04 pm

No transducer technology here, but I did have him measure pedal ratio and either he measured wrong or we may have found the problem because he has 3:1. He said the brake pedal has been cut and welded as if someone shortened it. The measurements were 9 and 3.
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