KnifeySpoony Posted October 10, 2022 Share Posted October 10, 2022 I have the AP Racing "race" cap with the bellows and no sensor- installed from new, as recommended to reduce/eliminate leaking from stock cap. It didn't leak a drop until last trackday it was leaking something fierce. Not sure what changed. I have a towel zip-tied over the cap as a precautionary measure to absorb any seepage. After each session on track Thursday, the towel was fully saturated and there was mist/spray around the area under the bonnet (presumably flinging off the wet towel). Changing out for a new towel each session would just result in another saturated towel. Made sure it was quite tight (can it be too tight?). Made sure the bellows was well-seated into the cap before screwing it on. Tried putting the bellows down into the reservoir, and then applied the cap. Nothing helped. I know some people on some cars recommend running much less fluid in the reservoir (say, only 50% full) to minimize leaks. I have always kept it topped up to "full", however over the course of the day, it ran down to about 75% full with no less leaking. Just now I cut a small ring of paperboard (cereal box lol) to fit into the ring groove in the cap to give just a little more downward pressure on the bellows flange to see if it helps seal more. Maybe something thicker would be of more help. Won't know until next time out on track next Saturday. It doesn't leak with street driving. Anyone dealt with this issue? Any hot tips? Link to comment Share on other sites More sharing options...
MV8 Posted October 10, 2022 Share Posted October 10, 2022 (edited) Caps don't need special sealing. Older models have no bellows; just a small hole in the cap. It sounds like the glycol fluid is boiling from moisture content. The fluid should be fully changed every season for racing (calipers inverted to drain if necessary). The fluid should meet DOT4 for a higher dry (and wet) boiling point. There are special glycol racing fluids (typically 550f) that exceed silicone DOT5 boiling point. DOT5 can result in a spongy pedal at high temps but is otherwise ideal for non-racing applications. The difference between wet and dry boiling point can be over 100f. Edited October 10, 2022 by MV8 Link to comment Share on other sites More sharing options...
sf4018 Posted October 10, 2022 Share Posted October 10, 2022 I use Millers Oils Racing Brake Fluid 300. It hasn’t boiled at all in 4 years. It has a boiling point of 310C. Link to comment Share on other sites More sharing options...
KnifeySpoony Posted October 10, 2022 Author Share Posted October 10, 2022 Fluid is fresh rbf 660. Pedal rock solid all day, don't think it's boiling. Link to comment Share on other sites More sharing options...
IamScotticus Posted October 10, 2022 Share Posted October 10, 2022 Vibration? Link to comment Share on other sites More sharing options...
MV8 Posted October 10, 2022 Share Posted October 10, 2022 (edited) In that case, after a run, I'd walk around checking the hubs for uniform heat from side to side with my palm. Maybe something is dragging (which can be hard to tell in such an overpowered machine) that is dumping more heat than normal into the fluid. Meanwhile, I'd run less than 50% in the reservoir and extend the bellows to displace some air over the fluid. I assume no brake lines are near the header or exhaust. Edited October 10, 2022 by MV8 Link to comment Share on other sites More sharing options...
KnifeySpoony Posted October 10, 2022 Author Share Posted October 10, 2022 No vibration. I did check hub/caliper temps with IR gun after several sessions on Thursday, as I had just replaced the RR caliper due to a leaking piston seal and wanted to make sure I was getting equal braking on both sides. Both rears were equal in temp- back of pads was about 450F on both sides hot off track iir. Fronts are less than half that but I figure that's to be expected with a solid disc and no airflow. Link to comment Share on other sites More sharing options...
panamericano Posted October 10, 2022 Share Posted October 10, 2022 "I'd walk around checking the hubs for uniform heat from side to side with my palm." Sound advice anytime, except for the poor palm. Better to use the back of the hand as the reflex, if hot, will pull away faster. Link to comment Share on other sites More sharing options...
ashyers Posted October 11, 2022 Share Posted October 11, 2022 I'd suggest checking the timing of the fluid intake/return port. If the seal displaces a large amount of fluid before passing over the port it will create a jet of fluid in the reservoir. This may lead to some leaking. Pop the top and have someone depress the peddle while watching the behavior of the fluid in the reservoir. If you get a geyser you may have an issue. Andy Link to comment Share on other sites More sharing options...
KnifeySpoony Posted October 11, 2022 Author Share Posted October 11, 2022 1 hour ago, ashyers said: I'd suggest checking the timing of the fluid intake/return port. If the seal displaces a large amount of fluid before passing over the port it will create a jet of fluid in the reservoir. This may lead to some leaking. Pop the top and have someone depress the peddle while watching the behavior of the fluid in the reservoir. If you get a geyser you may have an issue. Andy Will do - Is this something that could change/develop over time? Or something intrinsic to the master cylinder? Link to comment Share on other sites More sharing options...
MV8 Posted October 11, 2022 Share Posted October 11, 2022 Which end locks up first? It sounds like the rear was upgraded with no changes to the front and/or balance adjustment to compensate. I don't know your car's history of modification and caterham doesn't share much info on parts used. There is an optimal curve for the overall combination that shows line pressure versus G in deceleration and it is a curve versus a straight line. To optimize the brake system, a balance bar and an adjustable proportioning valve is usually needed. Provide the following info and I'll do some calcs: Curb weight, front/rear balance, cg height (if you have it), tire size(s), brake rotor diameters, pad types, if the calipers slide/float (pistons on one side) or are fixed (pistons on both sides), number and precise size of pistons, and master cylinder bore size. With no pressure applied to the brake pedal, the piston in the master should be seated against the snap ring. This is to ensure the tiny compensation port is open. A brief squirt of a few cc's is normal and necessary for a self-bleeding master. If bleeding, leave the cap on loosely or roll slowly into the pedal to prevent the geyser. Link to comment Share on other sites More sharing options...
ashyers Posted October 11, 2022 Share Posted October 11, 2022 Quote 10 hours ago, KnifeySpoony said: Will do - Is this something that could change/develop over time? Or something intrinsic to the master cylinder? It is intrinsic to the master. Often issues pop up if it's been rebuilt and clearances have not been maintained. Link to comment Share on other sites More sharing options...
KnifeySpoony Posted October 11, 2022 Author Share Posted October 11, 2022 5 hours ago, MV8 said: Which end locks up first? It sounds like the rear was upgraded with no changes to the front and/or balance adjustment to compensate. I don't know your car's history of modification and caterham doesn't share much info on parts used. There is an optimal curve for the overall combination that shows line pressure versus G in deceleration and it is a curve versus a straight line. To optimize the brake system, a balance bar and an adjustable proportioning valve is usually needed. Provide the following info and I'll do some calcs: Curb weight, front/rear balance, cg height (if you have it), tire size(s), brake rotor diameters, pad types, if the calipers slide/float (pistons on one side) or are fixed (pistons on both sides), number and precise size of pistons, and master cylinder bore size. With no pressure applied to the brake pedal, the piston in the master should be seated against the snap ring. This is to ensure the tiny compensation port is open. A brief squirt of a few cc's is normal and necessary for a self-bleeding master. If bleeding, leave the cap on loosely or roll slowly into the pedal to prevent the geyser. Front locks up first. I put higher friction pads on rear to move the bias rearward, as the front were overbraked with the uprated front calipers/larger rotors. I previously did the calculations on brake bias but don't have the numbers anymore. The stock system is heavily front-biased. A prop valve will not help without first upgrading the rear setup to overbrake the rear. A dual master/balance bar setup is ideal for sure, but I'm not ready to take on that project at this time. Link to comment Share on other sites More sharing options...
sf4018 Posted October 12, 2022 Share Posted October 12, 2022 If the rears are 450F and the fluid can take up to 300F it could be the combination of solid rotor and high friction pads aren’t going to work together without bubbling up the fluid in the caliper a little. As the rear pads wear there will be more fluid in the caliper to overheat, so it’s possible it gets worse over time. Link to comment Share on other sites More sharing options...
KnifeySpoony Posted October 12, 2022 Author Share Posted October 12, 2022 My fluid is rated for 617F boiling point. True, as pads wear thin, fluid will be more likely to boil. I would just expect to feel a change in pedal feel. But maybe that's an incorrect assumption. Link to comment Share on other sites More sharing options...
MV8 Posted October 12, 2022 Share Posted October 12, 2022 I'd put normal pads back on the rear to see if that has any bearing on the expansion overflow issue. I'd rather fit an adjustable prop than the temp variable of special pads. Besides under car ducting and/or water misters, caliper pistons can be swapped for phenolic (if available) or insulated from the pad backing plate to reduce the heat transfer into the caliper body and fluid. The problem with phenolic is it can eventually swell from moisture absorption from irregular use and storage in a humid environment but that should not be an issue with a race car in a controlled environment. Link to comment Share on other sites More sharing options...
KnifeySpoony Posted October 16, 2022 Author Share Posted October 16, 2022 Drove Sonoma yesterday. Had zero leakage. Between the thin paperboard ring and being careful not to overtighten, problem solved. I did a double thick towel over the top and a full diaper system underneath, and everything was bone dry. 2 Link to comment Share on other sites More sharing options...
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