Brake Pedal Feel

[ehendrikd]

Hi everyone

Just wondering if anyone has any previous experience or insight into the brake pedal feel I am currently experiencing. The car is an AE86 with the T3 Wilwood big brake kit + ST141 Corona Avante disc and drum handbrakes adapted to a YR22 Tarago F-series rear axle, with an NG Pajero 15/16" master cylinder and Wilwood adjustable proportioning valve. My assumption was even though the T3 kit is designed for the standard master cylinder that I would need a larger bore for the larger rear discs.

The system is bled correctly, and all weeping leaks fixed. With the engine off, there is a firm, consistent pedal. While the engine is running, there is also a firm pedal, with an expected small amount of travel provided by the vacuum. The issue appears to be that the brakes will bite and begin to lock BEFORE the pedal becomes firm with the engine running, almost as though there is too much vacuum on the booster (engine is a Beams 3SGE). What it feels like is the braking is happening in the middle of the vacuum assisted pedal travel, not by modulating the pressure where the pedal becomes hard. This leads to a very vague feel as to where the brakes are biting, and jerky braking when heel and toeing. My thoughts are that I should perhaps try the original smaller bore master cylinder.

Any input appreciated.
Cheers
Evan

[Matt]

Not sure if this is helpful, but do you have the 1 way valve in between the booster and engine vacuume port?

[ehendrikd]

Thanks for the reply, yes a new one way valve is installed between the booster and the inlet manifold, with the arrow on the valve pointing towards the inlet manifold.

[Admiral Ackbar]

Smaller master will make the pedal softer with more travel, so the opposite of what you want. Which end is locking first? Do you know the piston sizes?

[ehendrikd]

The proportioning valve was initially set at 50/50 and was adjusted to about 70/30 where both front and rear seem to be locking (need to confirm this). I am fairly certain are the front calipers and that the piston diameters are 1.38". I will attempt to measure the Corona caliper piston size.

This may also be a misunderstanding of my concept of what is happening in the system, I assumed when the pedal begins to harden, this is where the pads begin to contact the discs. This does not appear to be the case in this situation. Ideally what I am looking to achieve is to have the brakes start to bite where the pedal starts to harden and am not overly concerned how a solution would effect the overall pedal travel.

[Admiral Ackbar]

Yes, your understanding is correct.

Could be master cylinder too small:
- It will have to travel a long way to displace enough fluid to move the pads to the disk, causing a low pedal.
- If is too small (relative to your piston sizes) it may generate enough brake pressure to flex your calipers and possibly expand your flexible brake lines (unless they are braided), leading to a poor, slightly mushy pedal feel, but not as bad as air in the system. However new wilwood calipers shouldn't flex much, not sure about your rears. This amount of flex will be easily visible.
- With no vacumm assist you may not be able to apply enough pressure to flex stuff. That might be why the pedal feels ok when the engine is off then goes bad with the engine running.
- If this is the problem the pedal will travel a long way, then be firmish but squishy with poor feel, but you'll only be able to press the pedal so far.

OR air in the system:
- If there is a lot of air in the sytem the pedal will go to the floor.
- A very small amount of air or bad fluid can can make the pedal feel squishy but the brakes will still work.

For trouble shooting purposes wind the porportioning valve all the way out, then fix the pedal feel, then use the valve to adjust bias. Make sure the factory proportioning valve has been removed and the new one is installed in the rear line, not the front.

[Skylar]

Sounds like my piece of shit. Stock silvia. Bought a BM44 to put onto the car for when I put the 310mm disk and 4 pot brakes on. It's a pretty common thing to upgrade the master along with the brakes to what car the brakes came from.

I like the feel of corolla brakes and it's as you describe, brake force is equivalent to brake pedal firmness. The stock silvia is more like brake force is equivalent to brake pedal travel. I hated that feeling but eventualy got used to it. (the corolla's gonna feel real bad when I drive it next.) So I put the big brakes on the front, haven't done the rear yet but forgot to swap out the master before bleeding the system up. What I noticed is the pedal is a lot softer and you have to apply force to the pedal to get the brakes to work. Took a week to get used to.

Essentially, what I done is the opposite of what guy do and upsize the master to get pedal firmness back. I've essentially "downsized" the master by putting more (smaller) pistons in the system and made the pedal softer, which is what you're after.

Go down to a 7/8th and see how it feels.

[Admiral Ackbar]

He has a 15/16 master cylinder and I think he needs to try a bigger one, not smaller.

The master cylinder converts force from the brake pedal into hydraulic pressure in the brake system. Pressure = Force / Area. The smaller the cross sectional area in the master cylinder, the higher the hydraulic pressure will be for a given pedal force.

The brake calipers convert hydraulic pressure in the brake lines into force against the brake pads. Force = Pressure X Area. A caliper with larger piston area will exert a greater force force a given hydraulic pressure.

It might sound like a good idea to have a really small master cylinder and massive calipers with large pistons all round to create a powerful brake system, but it won't work. If you have a 3/4 inch master cylinder (1.905cm) that will have a cross sectional area of 2.85 square cm. If the brake pedal moves about 30cm the master cylinder piston will move about 5cm (6:1 leverage ratio) and displace 5 x 2.85 = 14.25 cc of brake fluid. Now say you put 4 piston calipers with 38mm pistons on each wheel. That's 16 pistons with a total area of 129 square cm. When you press the brake pedal down 30cm, the 14.25 cc of displaced brake fluid will move the pistons 14.25 / 129 = 0.11cm or 1.1mm, not far enough to get the pads to the disks They will have a lot of force, but they won't move very far. It's like a really long lever, it can exert a great force but you have to push one end a long way to move the other end a short distance.

With a bigger master cylinder ehendrikd won't have to press the pedal down as far, and the calipers will flex less (less leverage), which will firm up the pedal. Or the problem could be he has air in the lines.

If you think this is all just theory, I used to have a car with dual masters and a balance bar setup. With both masters the same size (bought the car like that) I had the balance bar adjusted all the way over to the side with the front master, but still couldn't get enough front bias. To fix the problem I changed the front master for a smaller one, then I was able to centre the balance bar.

[ehendrikd]

Thank you both for the detailed replies. All flexible sections in the system have braided lines, so unfortunately I do not think flex will be an issue. I have currently used SuperCheap branded DOT4 brake fluid, simply because I have been though many bottles ensuring the custom lines are leak free (all brake lines are custom) so now bleeding better fluid through may make a difference. It is also still possible that there may be small air pockets that I just haven't been able to bleed out yet.

I intend to do some testing adjusting the proportioning valve at it's extremes to see if the incorrect feel can be dialed out, and to perhaps have the fluid professionally bled before looking into changing the master cylinder.

[cooter]

Hey look this is probably an oversimplified answer but iam having similar dramas with my ke running standard booster, standard mc, all braided lines, e series drums and jdm vented 86 fronts.

i found that when I changed the mc over from another ke70 the threaded rod that goes from the booster to the mc needed to be adjusted to make the calipers bite when the pedal got firm.

prolly isn't your problem but maybe worth a look