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GT40 Tech - Chassis,Brakes,Tires,&Wheels Chassis and Handling.

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Old 4th March 2011, 12:01 AM   #1 (permalink)
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Residual pressure valve - brakes

Ryan reported mid way through a good drive today that the front brakes seemed to suddenly lose stopping power. There are no leaks and the fluid is topped off in both reservoirs. The brakes feel firm when applied, as they normally do.

We have Wilwood residual pressure valves in the front and rear brake circuits, as recommended where the master cylinder is lower than the calipers.

I am wondering if the front residual valve has "jammed" and is interfering with the forward flow of brake fluid when the brakes are applied.

Has any one had any issues or experience with these residual valves failing, and when they do, what are the symptoms?

It is Wilwood part 260-3278. Blue colored, 2 lb pressure.
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Old 4th March 2011, 10:26 AM   #2 (permalink)
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Re: Residual pressure valve - brakes

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Originally Posted by CESLAW View Post
We have Wilwood residual pressure valves in the front and rear brake circuits, as recommended where the master cylinder is lower than the calipers.
Does this recommendation apply if the reservoir is higher than the calipers?
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Old 4th March 2011, 11:32 AM   #3 (permalink)
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Re: Residual pressure valve - brakes

Quote:
Originally Posted by CESLAW View Post
Ryan reported mid way through a good drive today that the front brakes seemed to suddenly lose stopping power. There are no leaks and the fluid is topped off in both reservoirs. The brakes feel firm when applied, as they normally do.

We have Wilwood residual pressure valves in the front and rear brake circuits, as recommended where the master cylinder is lower than the calipers.

I am wondering if the front residual valve has "jammed" and is interfering with the forward flow of brake fluid when the brakes are applied.

Has any one had any issues or experience with these residual valves failing, and when they do, what are the symptoms?

It is Wilwood part 260-3278. Blue colored, 2 lb pressure.
Chuck

I remember Dean Lampe having some issues with them on his first test drives with his GT. It is decribed in his test report, but may be just easier to give him a call.

IMHO the reco is just for the case when the reservoir is also lower than the caliper. If the reservoir is higher than the caliper than you should be fine with out. Of course the initial pedal feel will be softer and you will have a longer pedal travel without them. I personally donīt like the idea of having some springloaded valve inbetween the main and caliper. So i mounted the reservoirs as high as possible and took the inner parts out of the RVīs . This allows me to reinstall if necessarry. No test drive done yet.

TOM
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Old 4th March 2011, 02:38 PM   #4 (permalink)
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Re: Residual pressure valve - brakes

Tom and Terry:

Good observation regarding the location of the reservoirs. I should have thought that through. Following is the quote from Wilwood's web site. It does not mention the location of the reservoir. But logically with the reservoir higher, gravity will keep the resting pressures constant.

"Wilwood two pound residual pressure valves retain a minimum brake line pressure to help eliminate excessive pedal travel in both disc and drum brake systems. The two pound valve is used in disc brake applications where the master cylinder is mounted below the horizontal plane of the calipers and fluid drain back occurs from gravity and vibration, thereby causing excessive caliper piston retraction and a longer brake pedal stroke. The minimal two pound residual pressure prevents fluid from flowing back without causing the brakes to drag. Residual Pressure Valves are made from billet aluminum and color coded for easy identification."

I was at Run and Gun when Dean Lampe had this problem and know he pulled the residual pressure valve and removed the inner portion, then reinstalled it. It seemed to work fine after that. I dont recall what happened that prompted this action.

It just seems a bit odd that this device would fail, and if it failed, prevent the brakes from working. It would seem to me that if if failed the effect would be just the opposte: brakes would work but may require a bit more pressure initially to compensate for the 'backflow'.

That is why I was wondering if any one else had had a failure of this part and if so, how did it behave.

Like you, Tom, my reservoirs are well above the horizontal plane of the calipers.

Thanks.
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Old 4th March 2011, 03:32 PM   #5 (permalink)
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Re: Residual pressure valve - brakes

I use the 2# residual valves in a number of different cars - even my Factory Five Challenge Car and have had great success in maintaining a higher pedal..

I wonder if perhaps a piece of dirt or something else may have compromised the brake balance bar assembly... Floor mounted pedals are terrible for this, although implementing a hanging pedal system in an RCR would be just beyond very difficult without cutting up the alloy tub.

Another thing might be dirt or something else not allowing the piston for the front master cylinder to return all the way to the resting position. This would leave the return port to the reservoir covered. I had this happen once before on another car where the circlip that retains the piston actually got bent and the .5mm protusion deeper in the boor prevented the piston's full return and created a similar problem..
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Old 4th March 2011, 04:35 PM   #6 (permalink)
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Re: Residual pressure valve - brakes

Have you checked the basics first, ensure that both M/Cyl pushrods have freeboard, that the balance bar locknut has not loosened and shifted the bias toward the rear M/Cyl
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Old 4th March 2011, 05:29 PM   #7 (permalink)
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Re: Residual pressure valve - brakes

I'm not aware of any issues with the Wilwood residual valves, save a few people that have installed them backwards.
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Old 4th March 2011, 08:51 PM   #8 (permalink)
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Re: Residual pressure valve - brakes

Jac:

As usual, you are on top of things. That was my first thought. Spent a fair bit of time head buried under the dash, butt hanging over the left sponson, checking the balance bar. All seems to be well in that department.

Ryan's description also suggested to me something other than a balance bar issue, since the change in brake function occurred relatively suddenly while on his afternoon drive.

Unfortunately we will be socked in with rain this weekend so I won't be testing it myself for a bit.

Still seems odd to me that those little residual valves would fail.



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Have you checked the basics first, ensure that both M/Cyl pushrods have freeboard, that the balance bar locknut has not loosened and shifted the bias toward the rear M/Cyl
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Old 24th March 2011, 05:15 PM   #9 (permalink)
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Re: Residual pressure valve - brakes

The answer is YES, they do fail.

We replaced the residual valve with a couple of fittings. Now the brakes work just fine.

But a correction: not sure the original residual valves were manufactured by Wilwood. We bought a couple from Wilwood just in case we decided to replace them and noted they were markedly different. Don't want to cast any aspirsions on Wilwood.

The old residual valve had no resistance when subjected to the 'blow' test from either end. I am guessing that the spring loaded flapper broke loose and when brake fluid flowed it interfered with the down stream flow.
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Old 24th March 2011, 08:06 PM   #10 (permalink)
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Re: Residual pressure valve - brakes

Chuck

How is the pedal feel and travel compared to before with the functioning RVīs?

TOM
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Old 24th March 2011, 11:29 PM   #11 (permalink)
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Re: Residual pressure valve - brakes

Tom:

Have not yet driven it long enough or hard enough to give a good assessment. Initial impression from a short drive with some good hard stops is that there is a bit less pedal resistance and a bit more pedal range. Honestly it has a better, more "controllable" feel if that makes sense. The difference is small and would be hard to notice if one did not drive it back to back with and without. At this point I would give the preference to leaving the residual valve out.

This weekend we plan to remove the rear residual valve and then go for a good long drive, weather permitting. Will give you an update then.
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