Page 1 of 1

#1 Zeus single pot. Pistons not retracting

Posted: Sat Jul 13, 2019 9:22 am
by Grayjay66
66 fhc, I replaced my original front left calipers with Zeus single pot stainless leaving original Dunlop on right front.

Simple bolt off and then Zeus bolt on . All seemed well. Drove around 6 miles and found Zeus side hub warm. Thinking I had a bearing problem, put the car up, took tire off and found Zeus wheel locked up , could not turn by hand. Original Dunlop side wheel turns with light touch. Zeus Pistons are not retracting.

Loosened outboard Zeus pot 4 bolts, wheel turning with drag, loosened inboard side and wheel turns freely.

Took both pots off, checked and I could hand press the Pistons back. Remounted , wheel turning freely. Bled brake line and wheel locked up again with pedal pressure established.

So since the Dunlop side is behaving normally and the Zeus side is not, it would seem the problem is with both of the Zeus calipers/pistons or I have a one way block in the brake line somewhere between the calipers and the line T to the right front. I did replace flexible section last Nov.

Any ideas as to cause?

#2 Re: Zeus single pot. Pistons not retracting

Posted: Sat Jul 13, 2019 9:28 am
by abowie
Grayjay66 wrote:
Sat Jul 13, 2019 9:22 am


So since the Dunlop side is behaving normally and the Zeus side is not, it would seem the problem is with both of the Zeus calipers/pistons
Sounds like it. Are they new? Used? Rebuilt?

#3 Re: Zeus single pot. Pistons not retracting

Posted: Sat Jul 13, 2019 9:38 am
by Grayjay66
Brand shiny new Zeus. Original du lops on right side. I bought a second pair of Zeus but now will not put on until this is resolved.

#4 Re: Zeus single pot. Pistons not retracting

Posted: Sat Jul 13, 2019 10:00 am
by abowie
Sorry but when I originally read your post I thought you had put a pair of Zeus calipers on the front.

You can't run different calipers on different sides; it doesn't work like that.

#5 Re: Zeus single pot. Pistons not retracting

Posted: Sat Jul 13, 2019 12:07 pm
by Doug Buchan
"You can't run different calipers on different sides; it doesn't work like that."

I think he means that he changed the cylinders on one side to Zeus which wouldn't be advisable IMO.
Zeus clearly states they are only sold in sets of four.

#6 Re: Zeus single pot. Pistons not retracting

Posted: Sat Jul 13, 2019 1:22 pm
by Grayjay66
The Zeus piston not retracting on the left front while the dunlops are on the right front, has nothing to do with the piston manufacturer being mixed .

I intended to change out both sides but was testing the Zeus before committing to the second pair, and was not concerned about the test drive as the swept pad area is exactly the same. Mis matched swept pad area is a very bad idea., unless you are driving a dirt track sprint car.

I forgot to mention once the Zeus locked up, even after opening the Zeus side bleed nipple, the Pistons would not release, leaving the wheel locked up.

#7 Re: Zeus single pot. Pistons not retracting

Posted: Sat Jul 13, 2019 1:51 pm
by Grayjay66
Also to clarify, the Zeus are single piston 2 1/8th as are the original dunlops.

#8 Re: Zeus single pot. Pistons not retracting

Posted: Sat Jul 13, 2019 4:27 pm
by Geoff Green
You have done the testing I would to determine what components are causing the problem. Time to call the vendor or manufacturer. Perhaps the piston seal (square cut o-ring) is installed backwards.

Pad swept area is different from piston diameter. Piston diameter determines the pressure and pad swept area determines the friction to stop. Both have to match side to side. I would not mix components on an axle.

#9 Re: Zeus single pot. Pistons not retracting

Posted: Sat Jul 13, 2019 5:02 pm
by Grayjay66
I agree about not mixing components although in this case piston diameter and pad area are the same so not a big concern for a short test drive.

I viewed it as a bit of an experiment to drive it with Zeus on one side and Dunlop on the other just to see if it did result in pull to one side or the other ( differential friction). Stopping From 40 mph there was no pull.

Although when I returned home the Zeus side hub was warm. Zeus do not have a positive retraction spring as do the Dunlops. And as I found the Zeus are binding.

Thank you for suggestion of the possible internal seal problem.

#10 Re: Zeus single pot. Pistons not retracting

Posted: Tue Jul 23, 2019 11:44 pm
by Grayjay66
I redid all diagnostic checks and found this problem is NOT the zeus brakes.

Vacuum system has some serious problems and likely affecting the reaction valve.

Will post again when I've done fixes.

#11 Re: Zeus single pot. Pistons not retracting

Posted: Thu Jul 25, 2019 7:35 pm
by Grayjay66
Spoke too soon. Vacuum leaks were not the fix I hoped for.

Fundamental question is why after opening the bleed nipple the brakes remain locked up, pistons not releasing force on pads?

#12 Re: Zeus single pot. Pistons not retracting

Posted: Thu Jul 25, 2019 8:29 pm
by 1954Etype
Check flexible brake pipes aren’t collapsing internally

#13 Re: Zeus single pot. Pistons not retracting

Posted: Thu Jul 25, 2019 9:52 pm
by abowie
Grayjay66 wrote:
Thu Jul 25, 2019 7:35 pm

Fundamental question is why after opening the bleed nipple the brakes remain locked up, pistons not releasing force on pads?
WRT the reaction valve. The small piston in the end of the can get sticky and bind and this can prevent the pressure in the brake system dropping back to zero, with the result that the pistons on the brakes don't fully retract and the brakes drag. The fix is to take off the reaction valve assembly, carefully remove the piston and liberally coat it with rubber grease or similar before putting it back. If this was your problem I would have expected it to have happened with you Dunlops as well.

A vacuum leak per se shouldn't make your brakes stay on. If you don't have vacuum in the system the reaction valve can't actuate the booster properly. As a result you have no vacuum assist on the brakes which will give you a heavier pedal as your foot has to do all the work. The car is quite driveable like this and the brakes will work adequately but it is a lot more effort.

Replacing the two front flexible lines as mentioned above is probably worth it, just in case there is a problem with them, but again, it should have been happening with your Dunlops as well.

If you release the hydraulic pressure inside the Zeus caliper cylinder by opening the bleed nipple and the brakes are still locked up there must be a problem with the piston being stuck in the bore somehow.

What was your reason for changing to the Zeus pots in the first place? Did you have a problem with your original brakes?

#14 Re: Zeus single pot. Pistons not retracting

Posted: Fri Jul 26, 2019 4:59 am
by Grayjay66
Thank you for the reply.

Irrespective of any other problems with master cyliner or flex lines it would seem opening the Zeus bleed nipple will release any hydraulic pressure on the Pistons and allow the wheel to turn. They remain stuck ..

So the Zeus Pistons are sticking in an unpressured system but why?

I'm thinking now it could be the Zeus have a thicker minimum pad Requirement. The pads on it now are worn but still thicker than the manual recommended minimum. It could be the Pistons are moving too far out of the cylinder and this cants the Pistons enough to prevent even slight retraction.

I am in contact with Zeus so will learn minimum recommended pad thickness and type. People seem to like the green stuff pads.

#15 Re: Zeus single pot. Pistons not retracting

Posted: Fri Jul 26, 2019 8:11 am
by abowie
Grayjay66 wrote:
Fri Jul 26, 2019 4:59 am

I am in contact with Zeus so will learn minimum recommended pad thickness and type. People seem to like the green stuff pads.
Good plan.

I like Greenstuff pads too.

#16 Re: Zeus single pot. Pistons not retracting

Posted: Wed Jul 31, 2019 9:48 am
by Grayjay66
I have been in contact with Zeus. They have been very responsive.

One last possibility was Dunlop spring return putting enough pressure to lock up Zeus. Eliminated this by disconnecting brake line inlet to Zeus side. Disc still locked up.

Returning set now for their eval.

#17 Re: Zeus single pot. Pistons not retracting

Posted: Thu Aug 01, 2019 7:46 pm
by scarr27
Just wondering if the problem might be using old pads (worn to suit a previous Dunlop cylinder) with a new Zeus cylinder? What I mean is that I have seen old pads which, for some reason, take on a slightly wedge shape after bedding in. Possibly due to the fact that the face of the piston is not exactly parallel to the face of the disc or the disc has worn unevenly. With this in mind, a new cylinders piston might not remain parallel in its bore if it were pressing against a worn, non parallel, pad back face. Could this cause the piston to cant over in it's bore and jam I'm wondering. I realise that the cylinders should be exactly parallel to the face of the disk when all is new but discs can wear unevenly. Just a though I had. :scratchheadyellow:

#18 Re: Zeus single pot. Pistons not retracting

Posted: Tue Aug 13, 2019 12:30 am
by Grayjay66
I thought the problem could be related to worn pads . I put new ones in with no resulting change. Pads remain in contact with the disc after brakes are applied and released. Using my trusty fish scale it looks like the friction can be overcome with 2-3 pounds constant force applied at the tire tread. But the pads do not lift off the disc entirely. Also checked cross over pipe which was clear.

Decided to give up and return to vendor. Shipping back to UK plus time waiting is prohibitive. A simple 2 hr job has turned into two months.

Now we'll see how the vendor treats me!