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Hey guys- I need help diagnosing and fixing my brake system.. the car is in the non-traditional builds area: viewtopic.php?f=36&t=16309 , and I've been chasing brake demons for awhile; I seem to be out of my depth.. Here's what I have-

-Master cylinder is part # MC M39451 (aka) Wagner MC 108134 (most agree it's a 7/8" bore MC, but it's been listed as both 21.0 and 53/64")
-Front brakes are 'stock' 1966 Volvo 1800S discs/rotors/calipers
-Rear brakes are 'stock' Ford T-bird (1989-1997) discs/rotors/calipers
-System is now using DOT4 'plus' fluid
-All 4 brake hoses are now stainless steel
-The Volvo 1800S pedal ratio has been modified to (now) be approximately 6:1
-This is a manual setup; non vacuum-assist
-The car is about 2400lbs (and about 300hp- goes fast, stops slow)
-An adjustable proportioning valve (dial type) is plumbed into the rear circuit
-I'm 100% certain there is no air in the lines (also bench bled the MC prior to install)

The problem is the brakes are not as effective as they should be- put another way, they don't grab hard enough... it almost feels like the pads are coated with WD40, and they'll take their own sweet time to slow the car.. More pedal pressure is of course helpful, but it never seems enough... I can sort of get the rear wheels to almost lock up, but I have to really, really push hard on the already soft-ish pedal, which feels like it's near the bottom of it's travel..

So here's where I did some experimenting, which may be helpful.. I know you're not supposed to 'preload' the master cylinder, but I did just that, and I learned a few things.. I adjusted the brake rod to where the MC plunger was about 1/8" into it's travel, and I went around the block once.. I knew the pedal would lose it's travel and get harder and harder with each push of the pedal, but I wondered what, if anything would the brakes do with this preload on them? in short, they worked really well!.. The pedal of course firmed up, and the "WD40 on the pads" feeling went away- the brakes actually had good clamping force! After a short time, the MC loaded up and I lost pedal travel..

It was nice to learn that when there's enough hydraulic pressure in the system, the calipers/rotors seem to work really well.. now it seems a matter of getting the right pressure and pedal feel..

So what do I do now? What component is to blame for the poor performance? I'm here to see what you guys think.. My gut tells be it's a master cylinder size/stroke issue?.. My MC donor car was a little 1990's econobox with tiny four wheel disc brakes/calipers.. maybe this master cylinder isn't moving enough fluid for these four larger calipers? (I can't seem to find info on the stroke on this MC).. Maybe go up to a 15/16" bore MC to firm up the pedal (but then I need more pedal pressure, which I can't seem to generate enough of as it is?).. I can go to a remote reservoir power brake setup, but really, if I design what I have correctly, shouldn't I be able to get really capable non-power brakes dialed in? Frustrating..

I'm stumped. The car is fast, but I no confidence in the brake's ability to get me stopped in a panic.. any thoughts?

As always- thanks for your input

-ccrunner

_________________
Dean

1972 Honda N600 Restomod "ccrunner's N600 VFR800 repower"

1963 Volvo P1800 Restomod
http://locostusa.com/forums/viewtopic.php?f=36&t=16309

1959 Berkeley SE492 Restomod...
viewtopic.php?f=36&t=19397

"ccrunner's 1960 MGA coupe Restomod" found on MGExp.com

How old are the brake pads?

The reason I ask, is because I had similar brake issues on my project: Huge amount of pedal effort to get weak braking. Pedal was rock-hard. I tried smaller M/C bore with some improvement, but not much. Eventually I put some new pads in because the math (and other people's setups) said that I should have plenty of pressure. After that I instantly had good brakes. I'd guess the resin degrades over time sitting in a shop. Even as a mechanic I had never seen brake pads alone cause such an issue before. Usually friction issues are just glazed rotors or contaminated pads.
Cheers.

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Build log: viewtopic.php?t=9291

You may be onto something C10.. I may have in fact glazed them myself while testing a few days ago- I got them HOT!

I wonder though, even if I did glaze/overheat them, how did I get such good brake feel and performance when I overadjusted ('preloaded') the master cylinder during testing? I would think if they were glazed that it would not have offered such good 'bite' as I was able to achieve, even if briefly..

--ccrunner

_________________
Dean

1972 Honda N600 Restomod "ccrunner's N600 VFR800 repower"

1963 Volvo P1800 Restomod
http://locostusa.com/forums/viewtopic.php?f=36&t=16309

1959 Berkeley SE492 Restomod...
viewtopic.php?f=36&t=19397

"ccrunner's 1960 MGA coupe Restomod" found on MGExp.com

You may need some fine tuning to get the balance right, but I'm guessing your pads are too hard; either from glazing or by the choice of compound. Performance pads are usually hard and take a lot of heat abuse, but unless the rotors are very hot, they grab like they are coated with WD40. The warm up lap is not only for tires.
I have found that basic pads work better, but may not wear as well, simply because on the street, you never get the brakes hot enough.
BTW, I'm having a similar problem, unfortunately, Wilwood doesn't make a basic soft pad. But I'm still working on it.

Just saw your last post. You probably put heat into the rotors, by dragging the pads. Try left footing the brake pedal and see if you can replicate that.

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Martin


My build log:
viewtopic.php?f=35&t=14520&start=0
My build video:
https://vimeo.com/143524140 password "matovid"

Re-reading your first post I see you have braided lines and still have a soft/low pedal. With braided lines, glazed rotors/pads (and proper bore sizes), you should have a rock hard pedal with little travel. In my case I had virtually no pedal travel even while pushing as hard as I could. Have you used the brake spreadsheet that is floating around this website? It's the first step if you haven't used it yet (thanks to whomever made that.... it's very useful). I think because you have a soft/low pedal, you have a caliper bore size mismatch F to R. This causes only one end to have brakes because the smaller piston is squeezing as hard as it can, while the larger one hasn't even started. When you "preloaded" the M/C to test, you probably moved the overly large caliper pistons out, closer to the pads which allows you to use all 4 corners. Once you sort out the bore sizes, then worry about friction surfaces (you may have both issues).

Cheers.

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Build log: viewtopic.php?t=9291

Will rapidly pumping the brakes help any? I mean if you get off the brakes and then quickly re-apply?

_________________
Chuck.

“Any suspension will work if you don’t let it.” - Colin Chapman

Visit my ongoing MGB Rustoration log: over HERE

Or my Wankel powered Locost log : over HERE

And don't forget my Cushman Truckster resto Locostusa.com/forums/viewtopic.php?f=36&t=17766

The rear brakes are huge and you should be able to lock up the rears at will. Have you tried to bypass the proportioning valve?
Additionally, I suspect you have a front/rear mismatch. Using my Alpine as an example, 2400 pounds with basically 50-50 weight distribution. Front brakes are 2 1/2" calipers on 11 1/2" rotors. Rears are 1 3/8" calipers on 10" rotors. Still need a ton of proportioning to keep the rears from premature lockup.

BTW, there is a retired automotive engineer on the SAOCA site, "Barry" who spent about 20 years working with the vendors that provide finished brake hose. He claims the widely held belief that braided SS brake lines flex less and produce a firmer pedal is totally unsupported by fact. I think the problem with braided lines is the braiding process is simply a loose cover and the lining has to expand against it. In conventional hose, the reinforcement is molded into the line.

Bill

True, but saying that also implies that it's squishy, which isn't true either. We worked out the numbers for expansion with typical brake line pressure and it was a fraction of a percent. I've plumbed cars both ways, with hardline and flex line and would choose flex line for anything I build just for the ease of assembly.

It's also said that hardline is cheaper than flex line - yes... and no. If you plumb many cars with hardline, then yes it is cheaper, because the volume absorbs the price of the flaring tool. For one-off builds, buying pre-made flex line in the needed lengths is actually cheaper.

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test 1: wrap both hands around each flex line and have an assistant push the brake pedal. You shouldn't feel anything, other than maybe the line trying to straighten out some. If you feel the line swell, you could have a bad line. Or someone might have sold you lines made out of plain rubber lined braided tubing instead of the right stuff for brakes.

test 2: take the top off the master cylinder and use your turkey baster or brake syringe to remove most of the fluid. You want about 1/4" of fluid over the shallowest compensating port. Have your assistant push down on the brake pedal. You should see a small amount of fluid squirt out the compensating ports. (hard to tell if the reservoir is full) You should see a squirt, then there shouldn't be any more fluid coming out. If any does, the master cylinder is leaking internally.

test 3: put your foot firmly on the brake for 60 seconds or so. If the pedal sinks enough to notice, your master cylinder is leaking internally

test 4: if your prop valve is adjustable, crank it to its extreme each way and see if it changes the pedal feel.

test 5: check free play between the brake pedal and master cylinder. As a rule of thumb, the pushrod shouldn't have more than 1/16" of slack. If you can move it freely, it's tight enough.

(It does not seem to make any difference if I repeatedly pump the brakes).. --I will do the above tests and see what's to be learned.. Also, is there a way (maybe a math formula) to tell if I'm mismatched Front to Rear? Can you guys link me to the brake spreadsheet and I'll run those numbers (as I should have done a long time ago )

--C10CoryM's statement makes a lot of sense too-

"...I think because you have a soft/low pedal, you have a caliper bore size mismatch F to R. This causes only one end to have brakes because the smaller piston is squeezing as hard as it can, while the larger one hasn't even started. When you "preloaded" the M/C to test, you probably moved the overly large caliper pistons out, closer to the pads which allows you to use all 4 corners. Once you sort out the bore sizes, then worry about friction surfaces (you may have both issues)..."

If it turns out that I am in fact mismatched F to R, what's the typical fix?

Thanks guys-

--ccrunner

_________________
Dean

1972 Honda N600 Restomod "ccrunner's N600 VFR800 repower"

1963 Volvo P1800 Restomod
http://locostusa.com/forums/viewtopic.php?f=36&t=16309

1959 Berkeley SE492 Restomod...
viewtopic.php?f=36&t=19397

"ccrunner's 1960 MGA coupe Restomod" found on MGExp.com

Brake Component Calculator Spreadsheet: http://locostusa.com/forums/viewtopic.php?f=5&t=14424&p=153058

_________________
-Justin

"Orville Wright did not have a pilots license." - Gordon MacKenzie

excellent- thank you

_________________
Dean

1972 Honda N600 Restomod "ccrunner's N600 VFR800 repower"

1963 Volvo P1800 Restomod
http://locostusa.com/forums/viewtopic.php?f=36&t=16309

1959 Berkeley SE492 Restomod...
viewtopic.php?f=36&t=19397

"ccrunner's 1960 MGA coupe Restomod" found on MGExp.com

The reason I asked is......if your brake calipers aren't perfectly aligned with the rotor, then the pads/piston get pushed back further than they usually are. This requires more pedal when 1st applying the brakes, and will be soft as the pads don't mate flat to the rotor. By preloading the M/C, you effectively prevented the caliper piston from fully returning. Giving a quick pump, it prevents the fluid from fully returning to the M/C. Also, sloppy guide pins on the calipers can also give a similar feel.

I'm not saying that this is the problem you are seeing, but it is a perfect fit of your symptoms and experimentation.

_________________
Chuck.

“Any suspension will work if you don’t let it.” - Colin Chapman

Visit my ongoing MGB Rustoration log: over HERE

Or my Wankel powered Locost log : over HERE

And don't forget my Cushman Truckster resto Locostusa.com/forums/viewtopic.php?f=36&t=17766

Rx7locost has an excellent point - I had a similar problem once with calipers that weren't sliding fully & smoothly on their pins (GM calipers).

The result was that the calipers were staying over to (and returning to) one side of their travel. On application of the brakes, fluid first went toward the system trying to center up the calipers, and then to generating clamping force with whatever fluid volume was left over in the stroke. There wasn't much, and the brakes were woefully wimpy.

It also made the pedal noticeably spongy, despite there being absolutely no air in the system at all.

Cleaning up the slider pins & making sure the caliper slides freely might be a solution to your problem. Free, too!

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Scratch building, at continental-drift speed, a custom McSoreley-design framed, dual-Weber 45DCOE carburated, Zetec-engined, ridiculously fast money pit.

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Another idea would be to check brake temps. Use an IR pyrometer. Heat the brakes with a few hard stops and check the temps (move fast!) The fronts should be warmer than the rears...should be symmetric side-to-side. This will help tell what is really happening.

I initially thought it might be incorrect F-R piston sizes, but....i think if there was not enough rear, the car would still stop pretty well (fronts do 70% of the work) If you had too much rear, you would probably lock up the rears in your brake testing.

Misaligned calipers (as mentioned) can be an issue. Pads that are worn to a taper can cause the same issue.