You shouldn't need to set the
ip_forward = 1 unless the interface is acting as a NAT for the other devices, which shouldn't be the case if you've set them up as a bridge.
Here's my KVM server setup which has a bridge device,
br0, with the physical ethernet device,
eth0 + all the interfaces for the KVM guests.
$ brctl show
bridge name bridge id STP enabled interfaces
br0 8000.bcaec123c1e2 no eth0
virbr0 8000.52540003f256 yes virbr0-nic
So what's wrong?
Based on your description it sounds like you don't have routing rules to route the packets from one interface to the other.
Host with the bridge
Host with NIC that's member of bridge
$ ip route show
192.168.1.0/24 dev br0 proto kernel scope link src 192.168.1.200
192.168.122.0/24 dev virbr0 proto kernel scope link src 192.168.122.1
169.254.0.0/16 dev br0 scope link metric 1008
default via 192.168.1.1 dev br0
$ ip route show
192.168.1.0/24 dev eth0 proto kernel scope link src 192.168.1.218
169.254.0.0/16 dev eth0 scope link metric 1002
default via 192.168.1.1 dev eth0
However you're likely running into an issue with mixing the
tap0 device and the physical ethernet device,
eth0, into a bridge.
Tap devices in bridges
Given you're using a TAP device,
tap0 you'll likely need to configure your firewall to allow these packets to flow back and forth over the bridge.
Now set up the Linux firewall to permit packets to flow freely over the newly created tap0 and br0 interfaces:
$ sudo iptables -A INPUT -i tap0 -j ACCEPT
$ sudo iptables -A INPUT -i br0 -j ACCEPT
$ sudo iptables -A FORWARD -i br0 -j ACCEPT