Sudden loss of hydraulic brake booster assist radial pump maybe not turning

1989 XJ6
Hydraulic brakes always worked well. SLS self leveling suspension removed before I bought car.
Recently the brakes on highway failed mid trip, no noises, and no more brake boost only minimal braking with master cylinder and foot pressure.
Took time but made the adapters necessary for adding Wakeon Brake Pressure gauges
1 at BOOSTER reading inlet pressure
1 at VALVE BODY reading pump out pressure
All setup with 2 pressure gauges and GREEN reservoir all full and its 2 small diameter return lines disconnected from green tank and redirected to a clean empty container to examine the fluid.

Started car idled OK but NO PRESSURE readings at all on both gauges and no fluid-mineral oil coming out of any of the 2 return lines

I listened with stethoscope at radial pump and only hear the engine idling.
Shouldn’t I hear some reciprocating pumping sounds? Zip nothing. Seems to reinforce the pump is not turning.

My suspicion is pump is not turning and the timing chain drive somehow is not driving the front drive plate or engaged with pump and or the pumps 2 metal “dogs” studs on its drive plate that engage the engine drive plate maybe both failed?

Anyone have any ideas
1_Should I take the 3 bolts off the pump with lines still attached (probably hard to remove) and pry it outward and peek inside and look for the pumps 2 engagement studs?

2_Drive plate possible easy test if it locked in position and wont rotate then its engagement to timing chain should still be AOK correct??

3_With pump unbolted and pried away should I start engine and see the drive plate is turning? doing number 2 may make this unnecessary.

—Any ideas? Thanks to all.

I recently moved the radial pump for the hydraulic system from my 1989 XJ40 (donor car) to my 1988. I left the hoses attached, and removed the upper coolant hose. Unbolted the pump and pulled it free. That should allow you to check out the drive plate as well as allow you to determine what is and is not turning.
There is a conversion kit available from to allow you to completely remove the hydraulic system brake booster and replace it with a vacuum assist unit.

Thanks for info… will unbolt pump… would be sad if the car side DRIVE DISK is not working or loose…
I’m trying to get my CD rom service manual working the disk may have failed…
thanks again. Unbolted pump yes much easier if upper radiator hose is removed. DRIVE DISK on engine is loose. I don’t know the exact timing arrangement but seems and timing chain driven IDLER GEAR has shaft the pokes through timing cover through an oil seal and that shaft is threaded in center for bolt and 2 holes drilled to accept the 2 roll pins from the engine side DRIVER DISK. . DRIVER DISK in pinned and bolted onto engine driven SHAFT. From there, there is a plastic or lets say fiber plastic like phenolic or similar disk to couple to engine drive disk to radial pump drive disk. But it’s not turning…
Yes turns out Jag engineers did not have a strong enough Loctite on bolt (still has blue residue) holding engine side DRIVER DISK to timing chain driven SHAFT. That bolt loosened, and there are 2 roll pins keeping it from rotating but with a loose bolt the engineers calculated in error as the 2 small roll pins were not enough to take the pulsating load of the hydraulic pump which seems AOK. For book correct the timing cover may have to come off but if I can remove the stubs of the roll pins, they are probably loose, remaining in the engine side idler drive SHAFT, I can get new roll pins and better Loctite and reinstall. Sad this drive coupling is not visible so inspection is impossible without removing the pump. Be warned if you remove the bolt in the engine drive side and start the engine… 60psi or so oil will jet out and spray the fan and the whole engine compartment. Yes it happened and took hours to clean. Car is now back in garage with nose facing out for better access and plan somehow to remove the roll pins. I’m thinking a very small tap? say 4-40 may be able to screw in and or just try a very thin screw might remove them. That would be great.
Anyone have any experience or ideas I thank you in advance.

Always good to know at what mileage this misfortune occurred.

136000 miles not allot for a 33 year old car… its a bit odd the bolt loosened though there was not much blue loctite on it that I could see

2tonejag …

Read you post … twice … and I’m a little lost with your talk of “DRIVER DISK” and ROLL PINS, but hey, that’s just me.

The hydraulic pump we are talking about of course is engine driven. So if you don’t have pressure …

  1. The mechanical pump itself has somehow failed … The least of your problems as many of us have that relic laying around and would be more than happy to donate it to you.

  2. The internal drive behind the timing chain cover has somehow failed … Major problem because removing that is a serious job.

So if the area around the hydraulic pump is leak free than just remove the pump and install a blank plate. And you can do this because you’ve now decided to convert your brake system to a vacuum boost … :grinning:

I won’t even get into the advantages of this conversion (there is enough information online on this to earn someone a PhD). Believe me it’s a no brainer and you’ll be able to remove enough (now) useless
hardware to fill up your trunk … er… boot.

TOP TIP … You can of course purchase the conversion kit for around $600-700. Or you can purchase a Series 3 XJ6 vacuum booster for about $200 and have a conversion plate fabricated for about $50 or make it yourself with the detailed measurments that are available on this forum.

The down side of this conversion is no more leaking (weird green) fluid coming out of your reservoir and no more strange “Brakes don’t work” messages on your VCM (that is still working isn’t it?) But that’s a whole 'nother post.

Just got part numbers the drive disk is called DRIVE COUPLING in parts catalog EAC7639 and the 2 roll pins are called SPIRAL PINS there are 2 and the part number is EAC7643 and the holes for such pins are dead ended so no pieces go into timing cover pulled some out with piano wire will try some small taps for remaining pieces.
Will try and obtain and new Drive coupling… maybe Welsh Ent??
Thanks to all