Upgrading the rear brakes to discs
Having upgraded the Stag front brakes to four pot calipers with drilled and grooved vented discs, it was now time to start the rear upgrade. The kit I had purchased from Monarch consists of discs, single pot sliding pin calipers, backplates to replace the drum brake items, handbrake cable, Goodridge reinforced brake hoses and all bolts and fittings required for fitting, including 3mm wheel spacers.
The old drum brakes first needed stripping off the car. After removing the drum, the brake shoe retaining springs were levered off, followed by the shoes. After disconnecting the handbrake cable and brake hydraulics (placing plugs into the ends of the brake pipes to reduce fluid loss), the brake cylinders could be withdrawn. I have Limora hubs and driveshafts fitted, so the complete hub had to be removed to allow the backplate change. The M20 centre nut is tightened to 240Nm, but I was able to undo it with a ½in pneumatic impact gun, which is easier than trying to lock the hub and use a long breaker bar. The new backplate is thicker than the original, so there was reduced thread engagement on the trailing arm studs for the nylon section of the nyloc self-locking nuts to fully engage with the thread, but I found some cone locknuts that had a smaller locking section and that solved this issue.
At this stage the hub and backplate were just retained with standard nuts so that I could assemble the rear brake discs and calipers together with pads and check clearance for the wheel. The caliper has to be partially disassembled to allow pad fitting, so I decided to remove the pin bolts with the caliper on the bench and immediately struck a problem. The pin has a hex section machined onto the end so a spanner can be used to prevent pin rotation whilst removing the bolt, but I was unable to find a spanner to fit – the machined hex was around 14.5mm across the flats, but my 15mm spanner was too thick to fit into the available space and a thinner 9/16in spanner was too small across the flats. I solved the issue by locating an old correctthickness 9/16in spanner and grinding the jaws away slightly. I could then hold the pins and remove the bolt that had been assembled with Loctite or similar. The pins were given a liberal coating of brake grease prior to reassembly.
After fitting the disc to the hub, the run out was checked with a dial gauge attached to the backplate. This was a maximum of 0.03mm, well within tolerance. The calipers and pads were then attached and the wheels bolted into place with the 3mm spacers supplied in the kit. The righthand wheel/ caliper clearance was fine, but there was a slight touch on the left, so I decided to replace the Monarch 3mm spacers with 5mm ones to ensure that there was adequate clearance on both wheels. This may have been necessary as I have non-standard hubs and driveshafts fitted.
The combination of the 5mm
spacer plus the disc being thicker than the old hub reduced the engagement of the wheel nuts onto the studs. Although thread engagement was still about equal to a standard nut, I decided to replace the wheel studs with longer items. This involved removing both hubs because it was easier to replace the wheel studs on the bench.
I started by using a hydraulic bearing puller to push the studs out of the hub as they were a tight fit. This was a relatively quick process with minimal effort required, but a hydraulic seal failed on the puller after only three studs, so I was back to using a standard screw thread puller which took more time and required far more physical effort. The replacement studs were then pulled into place using a nut and spacer on the studs and a torque wrench set towards the maximum recommended for the wheel nuts; this correctly positioned the new studs.
The hubs were then refitted and the cone locknuts holding the hubs to trailing arms torqued to the recommended value. The discs were fitted, followed by the calipers – Monarch had again supplied socket head cap screws for attachment, but I chose to replace them with hex bolts that I had drilled for lockwires. The pads were fitted and the calipers reassembled using Loctite on the sliding pin setscrews. The flexible hoses were fitted and secured to the trailing arm around midpoint. The hub retaining nuts were also torqued up – the hub has to be locked in position in order to generate the 240Nm torque required by the M20 nuts, a figure that requires a good push on my largest torque wrench that is 600mm long.
The kit contained a replacement handbrake cable, a standard Ford item, and a set of instructions that clearly defined the modifications required to fit this. This involve cutting the inner cable in the position given in the instructions and then cutting 300mm from one side of the outer cable. The cutting of the outer cable allowed the ends of the inner cable to overlap by 150mm where the two supplied duplex clamps could be attached to join the cable. I found that the outer cable on the other side was too long to allow a nice smooth run, so cut a section off this side and refitted the inner cable so it could be joined with the supplied clamps. After trying to fit these clamps I was unhappy with their fit, feeling that they were too small for the 3mm diameter inner cable, so I fitted some different clamps that I had available which were later replaced with duplex items that I purchased for 3mm cable. Both outers were securely fixed to the trailing arms so they did not droop and get caught on road obstructions.
The final task was to bleed all four corners of the brakes, starting with the rears. I have silicone fluid from which some users report it is difficult to remove all the air. Previously
I have never experienced any issues and went through the complete bleeding process by pumping the brake pedal. After it appeared that all air had been expelled and I had tightened the bleed nipples, the pedal seemed a little soft. I took the car on a test drive and was not happy with the pedal feel, so left the car overnight to allow any trapped air to move, then repeated the bleeding process. A further test revealed little change – the brakes operated as they should, but pedal travel seemed soft and long as if there was still air in the system.
I put the car back on the hoist for a further examination and came to the conclusion that the positioning of the bleed nipples on the rear calipers may not be evacuating all the air. The rear calipers were removed and supported so the bleed nipple was at the highest point and bled using a vacuum bleed system. After refitting everything, a short road test confirmed a firmer pedal with less travel. Further road testing is now scheduled.