Should I be able to lock up my brakes?

[gte901m]

I replaced my front calipers with the larger piston bore 3/4 ton chevy calipers, and also added new brake lines and pads. I bled the brakes, and I am not able to lock the front brakes up - should I be able to? Stock J20, no lift, or larger tires.

[james1414d]

Larger piston calipers requre a larger piston master cyl.

[gte901m]

Well, let me rephrase the question - should a properly operating stock brake systme on a '80 J20 be able to lock up the brakes?

I agree, in theory, larger piston calipers need a larger master cylinder.

[beloth]

Well, let me rephrase the question - should a properly operating stock brake systme on a '80 J20 be able to lock up the brakes?

I agree, in theory, larger piston calipers need a larger master cylinder.

any brake system that is not "anti-lock" should lock up if you hit the brakes with enough force and speed....mine lock up all the time (all drums) when i mash the heck out of them when some dill hole cuts me off and i do not feel like probing the Great Googley MoogleyGreat Googley MoogleyGreat Googley MoogleyGreat Googley MoogleyGreat Googley Moogley end of their car....:drivin:

[Elliott]

Hydroboost it.... I'll guarantee you'll wonder why you didn't start with one.

[HeavyIron]

Hydroboost it.... I'll guarantee you'll wonder why you didn't start with one.

I agree!

[james1414d]

Hydroboost it.... I'll guarantee you'll wonder why you didn't start with one.


by itself as a remedy, its merely a band-aid. first, get the apropriate master cylinder.

don't get me wrong, hydroboost is great! however an inadequate brake system will only turn into an inadequate hydroboosted brake system.

even though pascal's law says "pressure=force divided by area, the hydrostatic pressure for your calipers was studied be highly paid engineers back in the day. id trust their work and swap master cylinders with my larger calipers. chances are, it will bolt directly to your booster (witchever you choose).

so, just to be clear, I have nothing against ellioitt's hydroboost(I've pasted the thread on other sites), there is really no substitute for a properly engineered hydraulic system.

ratios are somewhat hard to figure out( slave/master bore+stroke, pistion diameter etc.), so id say the best bet if you're installing larger calipers, swap the master cyl too.

to reiterate, elliott, your hydroboost kits kick *** and we are lucky you've done the leg work and offer them. however, and im sure you'd agree, hydroboost does not fix nor compensate for a poorly designed hydraulic system.

[gte901m]

Hydraboost is on my list, I just wanted to make sure I had all the air bleed out. I guess I don't, because whenI slam the pedal down it stops but they don't lock up.

[Rich88]

Having over-sized tires means there's more rotational force or torque that the brakes need to overcome in order to stop...or lock-up.

Years ago I went from stock to 31 inch tires (not a lot), and kept the stock brake system. I noticed right away that I had less stopping power, although acceptable. I could still lock 'em up, but more pedal was required.

So....if you can't lock 'em up then as was implied, you need to beef up something.

[FSJ Guy]

Again, as mentioned, when you go to a larger bore size caliper, the master cylinder should be upgraded, too, to a larger bore MC. Otherwise, you will need A LOT more pressure on the pedal to apply the same braking force as before.

The larger bore caliper takes more fluid (since it is larger!) and therefore the master cylinder must PUSH more fluid to have the same braking force (PSI) as before. Soo... in order to have the same performance as before, either there needs to be more fluid being pushed OR more pressure being applied.

[tgreese]

Ethan, I seems you have it backwards.

The cylinders work just like the hydraulics in a bottle jack - if you have a little piston pumping into a big piston, the force on the little piston is multiplied by the ratio of the two piston areas. For example, 100 lbs on a 1 inch diameter piston will make 1600 lbs of force on a 4" diameter piston - the force then goes like the square of the increase in piston diameter (4 times squared = 16x).

Certainly the volume of fluid needed to move the same distance will also increase by 16x. So, to move the wheel piston the same distance will require you to move the MC piston more. You could increase the size of the MC piston so that the pedal does not have to move any further than it did before, but then what's the point of the larger wheel piston? You're then applying the same amount of force to the brakes but using bigger pistons on both ends. This will not make the brakes work any better.

[gte901m]

Well I just finished bleeding my brakes again. That did the trick. There was more air in the system than I thought. I used a vacuum bleeding system. Front brakes lock up - I did several trials at 30mph. Should the rear brakes lock up? Time to start saving for that hydroboost!

[Chevelleguy]

Normally the rears will lock up first because of weight transfer under braking.

[gte901m]

Actually, the rears may have locked up. I wasn't really paying attention to them - I was focusing on the road. I'll go check again :thumbsup:

[gte901m]

Both fronts do lock up, rears do not appear to.

So either there is still air in the rear line somewhere, or the proportioning valve is not working right.

What do yall think?

[royaustin2000]

You really don't want the rears locking before the fronts, about 70-80
percent of braking is done on the fronts, If the rears lock first there's
a good chance you'll swap ends if the road is at all slick. the proportioning
valve should operate this way.

[Ristow]

the bigger caliper piston gives you a hydraulic advantage with the same m/c. you'll get a bit more pedal travel for the same brake application.

the brake systems were designed by highly skilled engineers,and knee-jerk premature lock-up was high on the list as well,and was figured in to the m/c selection.

[gte901m]

You really don't want the rears locking before the fronts, about 70-80
percent of braking is done on the fronts, If the rears lock first there's
a good chance you'll swap ends if the road is at all slick. the proportioning
valve should operate this way.

Good point. Sounds like my system is functioning as it should. I bled the rear brakes twice as long as the fronts. Vacuumed sucked for about a minute after the last air bubble, so I am pretty sure there is no air in the system.

Are the rear drums on an '80 J20 self adjusting, or do I need to pull the drum and adjust them?

[Am4wag]

No need to pull drums to adjust.Have a slotted hole at bottom of backing plate in which to go through w/a screw driver or brake spoon and adjust.

[gte901m]

No need to pull drums to adjust.Have a slotted hole at bottom of backing plate in which to go through w/a screw driver or brake spoon and adjust.

Thanks.

[tgreese]

Thanks.
But they are self-adjusting. If you back up at 5-10 mph and slam on the brakes a few times, that will adjust them.

[Elliott]

If you do have plans to hydroboost it down the road don't buy a new master cylinder now because if it fits your '81+ vacuum booster it's not going to fit the hydroboost. I have one ton master cylinders for the one ton HB's I'm selling currently. It is a good idea to have your whole brake system functioning well prior to adding more pressure to it, but if you've narrowed it down to the MC then you may not want to waste the cost for a new one. Maybe slap another used one in there for the time being if you are fixing to HB it down the road.