Meritor Braking Improvement

[kraig]

Mike- Thanks for your test and researching improvements.

I may have only 07 that has the ISL 425. I know WRV did make changes whenever. My master cylinder has the two caps like the 1.75 inch master cylinder. Maybe that is why my brakes work better. Sounds like the master cylinder would be easier to change first and see what happens.

[EngineerMike]

Kraig- yours is the only 07 425 I have heard of; might have been the prototype for installing that engine package.

[kraig]

Mike- I agree, it is probably a prototype. So far all parts follow the 08, except now the master cylinder.

[EngineerMike]

Kraig- you can tell from looking at it if you have the same M.C.- it has the piston diameter cast into the forward face. 50.80 = 2", 44.45 = 1.75"
I don't think the tank is indicative, only the casting.

[kraig]

Mike- Checked mine today and it is 2" bore. The Bosch # for the 2" MC is 5172A which I can't find. Did the person who changed theirs to 1.75 bore give you a Bosch#? There is 6172A with 1.75 bore, but they don't stock it and haven't sold one before.

[EngineerMike]

I can't find a Bosch number for the 1.75" M.C. I think the way this works is HydroMax is a brand sold by Bendix and by Bosch, with some parts interchangeable, and multiple models available. There is a two-bolt mount M.C. for HMax, and a 4-bolt (our version). The 2" are always 4-bolt, and maybe all the 1.75's also; there are definitely some smaller bore M.C.'s (like 1") for the HMax that are 2-bolt, and some different versions of the booster. There are also setups that have no differential switch between the two brake fluid output ports, ours does; some have a fluid sensor on the tank and some (ours) don't. Some of these are used in forklifts and some in front end loaders.

Trick is getting the right "drop in" unit. Al from San Diego found an outfit (FL I believe) that sells the 1.75" new for $315. I have ordered one. The only difference Al found (see photo in post #13 above) is the tank that comes on the 1.75 has two caps instead of one like ours, a shorter profile, and there is a baffle between the two fill sides. Al felt the baffle was a full height block, making his into two tanks, one for front axle & one for rear; don't know if he diagnosed his correctly. When ordering I asked about that, the guy opened one and said there is a notch in the baffle, so that once one side is full of fluid to the notch it spills over to the other side. I asked about process to swap the new tank out for our one-tank design and he described a special tool setup to preclude the springs inside the cylinder pressure galleries from moving when taking the old tank off & putting new one on; I decided that was a) lotta work, and b) don't want to risk having to rebuild a brand new M.C. cuz I screw it up. Basically the tank or a substitute is integral to the cylinder assembly, so cylinder either comes loaded w/a tank or w/a "remote tank plate" letting you put tank elsewhere, so the cylinder assembly is complete. To avoid complications, I'm going w/the new tank when it gets here. Should have it in hand late this week, I hope. The shorter profile tank should help slightly in filling & power bleeding, maybe.

When it arrives, and I've verified drop-in status, I'll post additional data on ordering including variables to confirm when ordering (don't want to post that till I can see its actually working bug free). Cost was $315 + shipping, about $340 to the west coast.

Obviously this is not an OEM confirmed solution, and does not explain the differences between problem coaches and non-problem ones. Perhaps there is a different pressure relief spring set or orifice that got loaded into the problem units. Since mine is relieving at ~1,000psi like the specs say, I'm concluding the 1.75" M.C. will be alright in application. Al has some miles on his swap, and will have ~500 more by this time next week. I want to drive his rig before actually doing the swap just to feel the results. I'll do this at the Lake Tahoe SoCal rally next week, some time after coffee in the a.m. and before happy hour in the p.m.

[Louie & Linda]

EMike, Any update on the MC swap to 1.75?

[EngineerMike]

I drove Al Goodman's 1.75" 2008 36-footer yesterday. Night & vs mine. Sweet braking, very satisfactory. Ordered the 1.75 and expect it in around the 13th or so.

Don't know if that will allow invoking the ABS on dry pavement, but I'm certain it would lock wheels on slippery pavement in a panic stop. Nice mod. It will entail a 6-wheel-off brake bleed after the swap as detailed above, so its not for those who don't want to deal w/the wheels. And I won't be able to do the swap then take it to a shop for the bleed as the coach will be w/out proper brakes till the bleed.

I will report more after I've done the swap, and post photos.

[kraig]

Mike-Have a question about your 2" master cylinder. Mine is a 2'' ok, but it has two fill caps and you said yours had one. I did check mine and there is no splash over divider in it, it is solid all the way to the top. The Bosch OEM number stamped on mine is 5172A. Could tell me if yours is the same?

As far as bleeding the front brakes, you shouldn't have to remove the front wheels, as it is easy as it easy to use extension and socket through the hand hole in the wheel.

[EngineerMike]

In further discussion here at the Tahoe rally, we've decided the effusive air problem is because of not pre-bleeding the new master cylinder. If I buy two brake line nipples w/short extensions, & add some line to a receptacle I should be able to pre-bleed the MC before attaching the front & rear brake lines, probably won't introduce much air to the lines, and should be able to do a straight forward bleed and get all air out.

Kraig- I'll see what I can discern from my tank when I'm under the hood next.

[Al Goodman]

I like many of you was dissatisfied with the Alpine’s braking system to the point I refused to let my wife drive the coach. I truly questioned whether she had enough weight / strength to stop the coach in a panic situation. I read the posts, reviewed the modifications others had done. I flushed and changed the hydraulic fluid, and brake fluid, and even change the front brake pads to a more aggressive set, all with little improvement. Therefore I decided on modifying the bell crank as others have done, with positive results. To that end I removed the bell crank, fabricated a new arm went by a friends machine shop to utilize his lath for cutting off the existing arm. It was at this point it was suggested to me to check with a brake expert - Chuck Neal who owns a shop that manufactures braking systems for everything from Nascar to Off Road Racing, and happens to be located near me in San Diego.
OK maybe there’s a better idea out there? Went by and explained the problem to Chuck, he felt the master cylinder used on the Alpine’s was the incorrect application. A two inch bore master cylinder’s application would be drum brakes where the amount of fluid being displaced was necessary for brake operation VS disc brakes that require a much lower volume displacement of fluid, however higher pressure to achieve braking (volume vs pressure). It was suggested I find a smaller bore master cylinder. Only one I could locate was only ¼” smaller which concerned me. Back to Chuck to question if it would really be a benefit. Off the “top of his head” he told me the improvement would be 25-30%. I accepted his wisdom and ordered the master cylinder. Installation of the MC was simple and straight forward, the bleeding of brakes was quite time consuming, the results very rewarding. Coach now stops with reasonable foot pressure, doesn’t want to “walk” at stop signs or in reverse, and I feel comfortable having my wife drive it (as does she).

The fine print/liability disclamer: The solution I’ve found for what I felt was a problem is not for everyone, if you’re happy with your brakes, I wouldn’t change a thing. If not this is one suggestion, which worked for me. I would urge you to check with your mechanic / professional of choice get and follow his advice. This is simply what worked for me.

After the interest I received regarding this at the Tahoe Rally, I did contact the shop where I located the MC and requested they make it simple to order for any Alpine owner to order one.
E-Mike has ordered one, and I’d suggest you wait until his final verdict is in. He has also suggested a method of bleeding the master cylinder prior to connecting it to the brake system, which should make the task easier. However, if you haven’t changed your brake fluid, this would however be a good time to do so.

Cost $315.00 + shipping. A 10% discount has been offered for orders of 10 or more.
Order Info:
Results for A SPECIAL ORDERS:Special order parts.

Al G.

[EngineerMike]

Desert Dog is Al Goodman who I reference in post#22. Coach drives (i.e. stops) nicely compared to mine. I want mine to be like his.
Supposably my new M.C. will ship to me tomorrow from FL, maybe here Tuesday-ish, and hope to install next week. The link Al furnishes in post#25 above was put up by the supplier so there would be a one-click link reference to the particular part/application for this change-out.

Note it shows a Bendix part, not Bosch, and comes w/the shorter two-cap tank & pressure differential switch in the cylinder between the two output ports. Whether a Bosch part is available is an open question. But as noted above, the HydroMax appears to be made or at least marketed by both companies, and the MC's appear to be interchangeable insofar as bolt-on swap. Longevity & such obviously remains to be seen. Don't know if it has Bendix cast into the cylinder where my 2.0" has Bosch; will report on such matters when data is available.

[Jim A]

Since neither EngineerMike nor Al provided the math, here it is:

The surface area of a circle, such as a piston in a M.C., is pi x R x R. (Pi times the radius squared.) The surface area of the 2" diameter piston is 3.142 square inches. The surface area of the 1.75" diameter piston is 2.405 square inches.

If you apply 100 pounds of force to the 2" piston, you will generate 100/3.14, or 31.8 pounds per square inch of fluid pressure. Applying the same 100 pounds of force to a 1.75" piston will generate 41.6 pounds per square inch. That is (41.6-31.8)/31.8*100%, or a 30.8% increase in fluid pressure, and therefore braking force. (Assuming brake pedal leverage and caliper piston diameter stay the same, which they will.) That's pretty close to the 25-30% "top of the head" guess. Hey, my high school math teacher was right, math does apply to the real world! Who knew???

No wonder Mike and Al noticed an improvement.

Your brake pedal will need to move farther in order to move the same volume of fluid, so you should expect more pedal travel when braking. (A LOT more if you don't bleed the brakes properly!)

Math lesson over....

[Dale777]

Jim A,

Excellent explanation of the math (and physics of force and fluid movement).