Drag car brake/proportioning question

demon322 said
The race car I bought recently does not have a proportioning valve for the brakes. Is this common? Does it need one? I haven’t had a chance to weigh it yet but it’s fairly light. 71 Demon. Fiberglass front cap, hood, deck, and doors. Lexan windows. 4 wheel disc.

it does have line lock installed and this fitting in the line going to the rear brakes which I am not sure what is.

Thanks all.

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It is complicated! That is why there are rooms in Detroit full of engineers who do nothing more than design brake systems. With four wheel disc brakes you do not necessarily need a prop valve. A prop valve reduces the brake pressure to rear drum brakes. A disc/drum setup needs a prop valve since disc brakes are linear and drum brake regenerate. With disc brakes in the rear you have a linear system all the way around so you do not need a prop valve.

BUT, that is only true if the brake system is properly designed. Seeing the picture you posted tells me that your system was built by a hack not a chassis shop. Which means that it most likely is not properly designed so you'll have to do some testing before you'll know what to do. The proper balance of braking between the front and the rear depends on weight distribution, how high the center of gravity is and the available traction at each end of the car. A drag car with skinny tires up front an big slicks in the back might want more brake force on the back than a passenger car. Nobody on this forum can help you since we have no idea what your weight distribution is or how high the CG is or what the available traction is. Any answers you get are just guesses since we don't have any facts.

Get the car running and driving and then see how the brakes feel. If one end of the car is doing most of the braking then you'll have to make an adjustment. The best way to adjust it is to put larger rotors on the end that you want to increase braking force. Or you can use calipers with larger pistons in them. Typically you want a 66/33 split of brake force front to rear but that ratio is dependent on the stuff I mentioned before.

Take care, try it out slowly and see how it goes. There are a lot of different rotors and calipers available so you'll most likely end up just mixing and matching to find the correct balance.