Residual Pressure Valves for 7 brakes?

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Talkiet
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Residual Pressure Valves for 7 brakes?

Post by Talkiet » Sun Mar 08, 2015 11:23 am

Been reading a lot and it seems that I really should have residual pressure valves in both the front and rear brake circuits. I don't think it's inherent in the Master Cylinders.

Any reason I wouldn't do this? I think it's likely to fix the "one initial spongy pedal" I've had since replacing the rear brake pads and installing a proportioning valve on the rear circuit.

Cheers - N

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techtoys
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Re: Residual Pressure Valves for 7 brakes?

Post by techtoys » Tue Mar 10, 2015 1:11 pm

From what I understand, there are two scenarios where you would want residual brake pressure valves, either inside the master cylinder or separate external units.

1. If you have drum brakes, they have return springs that tend to force the brake fluid back towards the master cylinder. Maintaining a residual pressure in the brake line helps overcome this, thereby avoiding a "long pedal" next time you hit the brakes. Maybe a 10psi residual pressure might be a normal amount to achieve this. It was often built-in if the master cylinder was designed to be used for a drum-brake vehicle, but the residual pressure valve might need to be added externally if the master cylinder wasn't designed with this in mind.

There is also some suggestion that drum brakes have a poorer type of one-way seal on the piston in the wheel cylinder. Keeping the internal brake fluid pressure higher than external air pressure helps keeps the rubber lip around the seal firmly pressed against the inside wall of the wheel cylinder, preventing air from entering.

2. If you have disk brakes AND the master cylinder is mounted low on the floor behind the pedals, it can be lower than the height of the caliper. A residual pressure valve of about 2ps might be used to prevent fluid from draining downwards to the master cylinder, resulting in a "long pedal" next time you hit the brakes. If you've got disk brakes and the master cylinder is high up near the top of the firewall, you're probably okay.

Apparently the type of o-ring seal on the piston of a disk brake caliper seals quite well in both directions without needing a bit of internal fluid pressure behind it.

Regards
Pete

Talkiet
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Re: Residual Pressure Valves for 7 brakes?

Post by Talkiet » Tue Mar 10, 2015 1:19 pm

Thanks - that seems to echo what I have been reading but I decided to go and see Les Hunter today and explain the problem and ask him to sell me the things that are most likely to fix it.

After figuring out that the replacement remote reservoir master cylinders wouldn't fit the current connections without a BIG hassle, he suggested a 2lb Residual pressure valve so I grabbed one of those to put in the rear circuit. I'll install that when the NPt-npt union arrives and hopefully that cures my rear sponginess.

IF so, there's a sprint on the weekend that I'll have a crack at.

[edit, yes the master cylinder is very low]

Cheers - N

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techtoys
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Re: Residual Pressure Valves for 7 brakes?

Post by techtoys » Tue Mar 10, 2015 1:44 pm

I guess we'll all be interested to hear how well it works. I can't see any harm in having a residual pressure valve in the line anyway.

And Les Hunter knows more about these things than I ever will.

Do you wind the proportioning valve to Full Pressure Rear before bleeding the rear brakes? I didn't on my car, and it didn't bleed the air out very well.

Talkiet
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Re: Residual Pressure Valves for 7 brakes?

Post by Talkiet » Tue Mar 10, 2015 1:50 pm

techtoys wrote:I guess we'll all be interested to hear how well it works. I can't see any harm in having a residual pressure valve in the line anyway.

And Les Hunter knows more about these things than I ever will.

Do you wind the proportioning valve to Full Pressure Rear before bleeding the rear brakes? I didn't on my car, and it didn't bleed the air out very well.
Yes, made sure the valve was at full pressure before bleeding...

Hopefully the union arrives from Akl tomorrow (NPT unions are rare) and I'll install tomorrow night. Will be a very quick and easy install with the union (no new pipes or flares needed) so might have something to report tomorrow night.

Cheers -N

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Re: Residual Pressure Valves for 7 brakes?

Post by Talkiet » Sat Mar 14, 2015 4:55 pm

Update... Now have perfect brakes...

COmbination of the 2lb residual pressure valve and a closed circuit bleeding... (I ran a long pipe from the bleed nipples at the back up to the master cylinder at the front and then slowly pumped until the tube was full and no bubbles... Did both sides like that, cleaned the engine bay of brake fluid, dried it, went for a test drive and it's all good. Firm pressure straightaway and setting the rear balance to be correct only took two attempts with the proportioning valve.

Looks like I'll be at Ruapuna tomorrow! Yay!

Cheers - N

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